describe: teach describe negative pattern matches

Teach git-describe the `--exclude` option which will allow specifying
a glob pattern of tags to ignore. This can be combined with the
`--match` patterns to enable more flexibility in determining which tags
to consider.

For example, suppose you wish to find the first official release tag
that contains a certain commit. If we assume that official release tags
are of the form "v*" and pre-release candidates include "*rc*" in their
name, we can now find the first release tag that introduces the commit
abcdef:

  git describe --contains --match="v*" --exclude="*rc*" abcdef

Add documentation, tests, and completion for this change.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1198,6 +1198,7 @@ _git_describe ()
 		__gitcomp "
 			--all --tags --contains --abbrev= --candidates=
 			--exact-match --debug --long --match --always
+			--exclude
 			"
 		return
 	esac