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 <andersk@ksplice.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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