gitk: Honor TMPDIR when viewing external diffs

gitk fails to show diffs when browsing a read-only repository.
This is due to gitk's assumption that the current directory is always
writable.

Teach gitk to honor either the GITK_TMPDIR or TMPDIR environment
variables.  This allows users to override the default location
used when writing temporary files.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
gitk: Use mktemp -d to avoid predictable temporary directories

gitk uses a predictable ".gitk-tmp.$PID" pattern when generating
a temporary directory.

Use "mktemp -d .gitk-tmp.XXXXXX" to harden gitk against someone
seeding /tmp with files matching the pid pattern.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
gitk: Catch mkdtemp errors

105b5d3f ("gitk: Use mktemp -d to avoid predictable temporary
directories") introduced a dependency on mkdtemp, which is not
available on Windows.

Use the original temporary directory behavior when mkdtemp fails.
This makes the code use mkdtemp when available and gracefully
fallback to the existing behavior when it is not available.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
gitk: Add visiblerefs option, which lists always-shown branches

When many branches contain a commit, the branches used to be shown in
the form "A, B and many more", where A, B can be master of current
HEAD. But there are more which might be interesting to always know about.
For example, "origin/master".

The new option, visiblerefs, is stored in ~/.gitk. It contains a list
of references which are always shown before "and many more" if they
contain the commit. By default it is `{"master"}', which is compatible
with previous behavior.

Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Merge git://ozlabs.org/~paulus/gitk

* git://ozlabs.org/~paulus/gitk:
  gitk: Add visiblerefs option, which lists always-shown branches
  gitk: Catch mkdtemp errors
  gitk: Use mktemp -d to avoid predictable temporary directories
  gitk: Honor TMPDIR when viewing external diffs
Sync with master

* master:
  gitk: Add visiblerefs option, which lists always-shown branches
  gitk: Catch mkdtemp errors
  gitk: Use mktemp -d to avoid predictable temporary directories
  gitk: Honor TMPDIR when viewing external diffs
gpg-interface: provide clear helper for struct signature_check

The struct has been growing members whose malloced memory needs to be
freed. Do this with one helper function so that no malloced memory shall
be left unfreed.
gpg-interface: provide access to the payload

In contrast to tag signatures, commit signatures are put into the
header, that is between the other header parts and commit messages.

Provide access to the commit content sans the signature, which is the
payload that is actually signed. Commit signature verification does the
parsing anyways, and callers may wish to act on or display the commit
object sans the signature.
verify-commit: scriptable commit signature verification

Commit signatures can be verified using "git show -s --show-signature"
or the "%G?" pretty format and parsing the output, which is well suited
for user inspection, but not for scripting.

Provide a command "verify-commit" which is analogous to "verify-tag": It
returns 0 for good signatures and non-zero otherwise, has the gpg output
on stderr and (optionally) the commit object on stdout, sans the
signature, just like "verify-tag" does.
t7510: test verify-commit

This mixes the "git verify-commit" tests in with the "git show
--show-signature" tests, to keep the tests more readable.

The tests already mix in the "call show" tests with the "verify" tests.
So in case of a test beakage, a '-v' run would be needed to reveal the
exact point of breakage anyway.

Additionally, test the actual output of "git verify-commit" and "git
show --show-signature" and compare to "git cat-file".
log: correctly identify mergetag signature verification status

A wrong '}' made our code record the results of mergetag signature
verification incorrectly.

Fix it.