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 new "sym" capability (e.g. "sym=HEAD:refs/heads/master"; 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()

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>
fixup! upload-pack: send symbolic ref information as capability
fixup t5505
Merge branch 'jc/upload-pack-send-symref' into pu

* jc/upload-pack-send-symref:
  fixup t5505
  fixup! upload-pack: send symbolic ref information as capability
  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()

Conflicts:
	cache.h