If abbrev is set to zero in git-describe, don't add the unique suffix

If abbrev is set to zero in git-describe, don't add the unique suffix

When on a non-tag commit, git-describe normally outputs descriptions of
the form
  v1.0.0-g1234567890
Some scripts (for example the update hook script) might just want to
know the name of the nearest tag, so they then have to do
 x=$(git-describe HEAD | sed 's/-g*//')
This is costly, but more importantly is fragile as it is relying on the
output format of git-describe, which we would then have to maintain
forever.

This patch adds support for setting the --abbrev option to zero.  In
that case git-describe does as it always has, but outputs only the
nearest found tag instead of a completely unique name.  This means that
scripts would not have to parse the output format and won't need
changing if the git-describe suffix is ever changed.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>