completion: complete refs for "git commit -c"

The "-c" and "-C" options take an existing commit, so let's
complete refs, just as we would for --squash or --fixup.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -971,6 +971,13 @@ _git_commit ()
 {
 	__git_has_doubledash && return
 
+	case "$prev" in
+	-c|-C)
+		__gitcomp_nl "$(__git_refs)" "" "${cur}"
+		return
+		;;
+	esac
+
 	case "$cur" in
 	--cleanup=*)
 		__gitcomp "default strip verbatim whitespace