blame: compute abbreviation width that ensures uniqueness

Julia Lawall noticed that in linux-next repository the commit object
60d5c9f5 (shown with the default abbreviation width baked into "git
blame") in output from

  $ git blame -L 3675,3675 60d5c9f5b -- \
      drivers/staging/brcm80211/brcmfmac/wl_iw.c

is no longer unique in the repository, which results in "short SHA1
60d5c9f5 is ambiguous".

Compute the minimum abbreviation width that ensures uniqueness when
the user did not specify the --abbrev option to avoid this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
push: don't guess at qualifying remote refs on deletion

When we try to push a ref and the right-hand side of the
refspec does not find a match, we try to create it. If it is
not fully qualified, we try to guess where it would go in
the refs hierarchy based on the left-hand source side. If
the source side is not a ref, then we give up and give a
long explanatory message.

For deletions, however, this doesn't make any sense. We
would never want to create on the remote side, and if an
unqualified ref can't be matched, it is simply an error. The
current code handles this already because the left-hand side
is empty, and therefore does not give us a hint as to where
the right-hand side should go, and we properly error out.
Unfortunately, the error message is the long "we tried to
qualify this, but the source side didn't let us guess"
message, which is quite confusing.

Instead, we can just be more succinct and say "we can't
delete this because we couldn't find it". So before:

  $ git push origin :bogus
  error: unable to push to unqualified destination: bogus
  The destination refspec neither matches an existing ref on the remote nor
  begins with refs/, and we are unable to guess a prefix based on the source ref.
  error: failed to push some refs to '$URL'

and now:

  $ git push origin :bogus
  error: unable to delete 'bogus': remote ref does not exist
  error: failed to push some refs to '$URL'

It is tempting to also catch a fully-qualified ref like
"refs/heads/bogus" and generate the same error message.
However, that currently does not error out at all, and
instead gets sent to the remote side, which typically
generates a warning:

  $ git push origin:refs/heads/bogus
  remote: warning: Deleting a non-existent ref.
  To $URL
   - [deleted]         bogus

While it would be nice to catch this error early, a
client-side error would mean aborting the push entirely and
changing push's exit code. For example, right now you can
do:

  $ git push origin refs/heads/foo refs/heads/bar

and end up in a state where "foo" and "bar" are deleted,
whether both of them currently exist or not (and see an
error only if we actually failed to contact the server).
Generating an error would cause a regression for this use
case.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name.c: get_describe_name() by definition groks only commits

Teach get_describe_name() to pass the disambiguation hint down the
callchain to get_short_sha1().

Also add tests to show various syntactic elements that we could take
advantage of the object type information to help disambiguration of
abbreviated object names.  Many of them are marked as broken, and
some of them will be fixed in later patches in this series.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name.c: get_sha1_1() takes lookup flags

This is to pass the disambiguation hints from the caller down the
callchain.  Nothing is changed in this step, as everybody just
passes 0 in the flag.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name.c: many short names can only be committish

We know that the token "$name" that appear in "$name^{commit}",
"$name^4", "$name~4" etc. can only name a committish (either a
commit or a tag that peels to a commit).  Teach get_short_sha1() to
take advantage of that knowledge when disambiguating an abbreviated
SHA-1 given as an object name.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name.c: teach lookup context to get_sha1_with_context()

The function takes user input string and returns the object name
(binary SHA-1) with mode bits and path when the object was looked
up in a tree.

Additionally give hints to help disambiguation of abbreviated object
names when the caller knows what it is looking for.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name.c: introduce get_sha1_committish()

Many callers know that the user meant to name a committish by
syntactical positions where the object name appears.  Calling this
function allows the machinery to disambiguate shorter-than-unique
abbreviated object names between committish and others.

Note that this does NOT error out when the named object is not a
committish. It is merely to give a hint to the disambiguation
machinery.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
revision.c: allow handle_revision_arg() to take other flags

The existing "cant_be_filename" that tells the function that the
caller knows the arg is not a path (hence it does not have to be
checked for absense of the file whose name matches it) is made into
a bit in the flag word.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
revision.c: the "log" family, except for "show", takes committish

Add a field to setup_revision_opt structure and allow these callers
to tell the setup_revisions command parsing machinery that short SHA1
it encounters are meant to name committish.

This step does not go all the way to connect the setup_revisions()
to sha1_name.c yet.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name.c: add support for disambiguating other types

This teaches the revision parser that in "$name:$path" (used for a
blob object name), "$name" must be a tree-ish.

There are many more places where we know what types of objects are
called for.  This patch adds support for "commit", "treeish", "tree",
and "blob", which could be used in the following contexts:

 - "git apply --build-fake-ancestor" reads the "index" lines from
   the patch; they must name blob objects (not even "blob-ish");

 - "git commit-tree" reads a tree object name (not "tree-ish"), and
   zero or more commit object names (not "committish");

 - "git reset $rev" wants a committish; "git reset $rev -- $path"
   wants a treeish.

They will come in later patches in the series.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
apply: --build-fake-ancestor expects blobs

The "index" line read from the patch to reconstruct a partial
preimage tree records the object names of blob objects.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
commit-tree: the command wants a tree and commits

Signed-off-by: Junio C Hamano <gitster@pobox.com>
reset: the command takes committish

This is not strictly correct, in that resetting selected index
entries from corresponding paths out of a given tree without moving
HEAD is a valid operation, and in such case a tree-ish would suffice.

But the existing code already requires a committish in the codepath,
so let's be consistent with it for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
rev-parse: A and B in "rev-parse A..B" refer to committish

Signed-off-by: Junio C Hamano <gitster@pobox.com>
rev-parse --disambiguate=<prefix>

The new option allows you to feed an ambiguous prefix and enumerate
all the objects that share it as a prefix of their object names.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'hv/remote-end-hung-up' into jch

When we get disconnected while expecting a response from the remote
side because authentication failed, we issued an error message "The
remote side hung up unexpectedly."  Give hint that it may be a
permission problem in the message when we can reasonably suspect it.

Will merge to 'master'.

* hv/remote-end-hung-up:
  remove the impression of unexpectedness when access is denied

Conflicts:
	t/t5512-ls-remote.sh
Merge branch 'mm/push-default-switch-warning' into jch

Hopefully we can have a solidly tested series early in 1.7.12 or
1.7.13 at the latest.

# By Matthieu Moy
* mm/push-default-switch-warning:
  push: start warning upcoming default change for push.default
Merge branch 'th/diff-no-index-fixes' into jch

"git diff --no-index" did not correctly handle relative paths and
did not correctly give exit codes when run under "--quiet" option.

Will merge to 'master'.

# By Jeff King (1) and Tim Henigan (1)
* th/diff-no-index-fixes:
  diff-no-index: exit(1) if 'diff --quiet <repo file> <external file>' finds changes
  diff: handle relative paths in no-index
Merge branch 'nd/clone-single-fix' into jch

"git clone --single-branch" to clone a single branch did not limit
the cloning to the specified branch.

Will merge to 'master'.

# By Nguyễn Thái Ngọc Duy
* nd/clone-single-fix:
  clone: fix ref selection in --single-branch --branch=xxx
Merge branch 'jn/perl-makemaker-leading-paths' into jch

Minor build tweak.  People who do not use ExtUtils::MakeMaker may
want to test this before it hits the 'master' branch.

# By Johannes Sixt (1) and Jonathan Nieder (1)
* jn/perl-makemaker-leading-paths:
  perl/Makefile: Fix a missing double-quote
  perl/Makefile: move "mkdir -p" to module installation loop for maintainability