describe: Delay looking up commits until searching for an inexact match
describe: Delay looking up commits until searching for an inexact match

Now that struct commit.util is not used until after we've checked that
the argument doesn't exactly match a tag, we can wait until then to
look up the commits for each tag.

This avoids a lot of I/O on --exact-match queries in repositories with
many tags.  For example, 'git describe --exact-match HEAD' becomes
about 12 times faster on a cold cache (3.2s instead of 39s) in a
linux-2.6 repository with 2000 packed tags.  That is a huge win for the
interactivity of the __git_ps1 shell prompt helper when on a detached
HEAD.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@ksplice.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git