t5505: fix "set-head --auto with ambiguous HEAD" test

When two or more branches point at the same commit and HEAD is
pointing at one of them, without the symref extension, there is no
way to remotely tell which one of these branches HEAD points at.
The test in question attempts to make sure that this situation is
diagnosed and results in a failure.

However, even if there _were_ a way to reliably tell which branch
the HEAD points at, "set-head --auto" would fail if there is no
remote tracking branch.  Make sure that this test does not fail
for that "wrong" reason.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
upload-pack.c: do not pass confusing cb_data to mark_our_ref()

The callee does not use cb_data, and the caller is an intermediate
function in a callchain that later wants to use the cb_data for its
own use.  Clarify the code by breaking the dataflow explicitly by
not passing cb_data down to mark_our_ref().

Signed-off-by: Junio C Hamano <gitster@pobox.com>
upload-pack: send symbolic ref information as capability

One long-standing flaw in the pack transfer protocol was that there
was no way to tell the other end which branch "HEAD" points at.
With a capability "symref=HEAD:refs/heads/master", let the sender to
tell the receiver what symbolic ref points at what ref.

This capability can be repeated more than once to represent symbolic
refs other than HEAD, such as "refs/remotes/origin/HEAD").

Add an infrastructure to collect symbolic refs, format them as extra
capabilities and put it on the wire.  For now, just send information
on the "HEAD" and nothing else.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
upload-pack: send non-HEAD symbolic refs

With the same mechanism as used to tell where "HEAD" points at to
the other end, we can tell the target of other symbolic refs as
well.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
connect.c: make parse_feature_value() static

Signed-off-by: Junio C Hamano <gitster@pobox.com>
connect: annotate refs with their symref information in get_remote_head()

By doing this, clients of upload-pack can now reliably tell what ref
a symbolic ref points at; the updated test in t5505 used to expect
failure due to the ambiguity and made sure we give diagnostics, but
we no longer need to be so pessimistic. Make sure we correctly learn
which branch HEAD points at from the other side instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
clone: test the new HEAD detection logic

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jc/upload-pack-send-symref' into pu

* jc/upload-pack-send-symref:
  clone: test the new HEAD detection logic
  connect: annotate refs with their symref information in get_remote_head()
  connect.c: make parse_feature_value() static
  upload-pack: send non-HEAD symbolic refs
  upload-pack: send symbolic ref information as capability
  upload-pack.c: do not pass confusing cb_data to mark_our_ref()
  t5505: fix "set-head --auto with ambiguous HEAD" test

Conflicts:
	cache.h