Use git-describe --exact-match in bash prompt on detached HEAD

Most of the time when I am on a detached HEAD and I am not doing
a rebase or bisect operation the working directory is sitting on a
tagged release of the repository.  Showing the tag name instead of
the commit SHA-1 is much more descriptive and a much better reminder
of the state of this working directory.

Now that git-describe --exact-match is available as a cheap means
of obtaining the exact annotated tag or nothing at all, we can
favor the annotated tag name over the abbreviated commit SHA-1.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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