Teach bash to complete ref arguments to git-describe

I'm often finding that I need to run git-describe on very long
remote tracking branch names, to find out what tagged revision
the remote tracking branch is now at (or not at).  Typing out
the ref names is painful, so bash completion on them is a very
useful feature.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

contrib/completion/git-completion.bash

@@ -525,6 +525,11 @@ _git_commit ()
 	COMPREPLY=()
 }
 
+_git_describe ()
+{
+	__gitcomp "$(__git_refs)"
+}
+
 _git_diff ()
 {
 	__git_complete_file
@@ -1065,6 +1070,7 @@ _git ()
 	cherry-pick) _git_cherry_pick ;;
 	commit)      _git_commit ;;
 	config)      _git_config ;;
+	describe)    _git_describe ;;
 	diff)        _git_diff ;;
 	fetch)       _git_fetch ;;
 	format-patch) _git_format_patch ;;
@@ -1112,6 +1118,7 @@ complete -o default -o nospace -F _git_checkout git-checkout
 complete -o default -o nospace -F _git_cherry git-cherry
 complete -o default -o nospace -F _git_cherry_pick git-cherry-pick
 complete -o default -o nospace -F _git_commit git-commit
+complete -o default -o nospace -F _git_describe git-describe
 complete -o default -o nospace -F _git_diff git-diff
 complete -o default -o nospace -F _git_fetch git-fetch
 complete -o default -o nospace -F _git_format_patch git-format-patch
@@ -1145,6 +1152,7 @@ complete -o default -o nospace -F _git git.exe
 complete -o default -o nospace -F _git_branch git-branch.exe
 complete -o default -o nospace -F _git_bundle git-bundle.exe
 complete -o default -o nospace -F _git_cherry git-cherry.exe
+complete -o default -o nospace -F _git_describe git-describe.exe
 complete -o default -o nospace -F _git_diff git-diff.exe
 complete -o default -o nospace -F _git_format_patch git-format-patch.exe
 complete -o default -o nospace -F _git_log git-log.exe