git-prompt: do not look for refs/stash in $GIT_DIR

Since dd0b72c (bash prompt: use bash builtins to check stash
state, 2011-04-01), git-prompt checks whether we have a
stash by looking for $GIT_DIR/refs/stash. Generally external
programs should never do this, because they would miss
packed-refs.

That commit claims that packed-refs does not pack
refs/stash, but that is not quite true. It does pack the
ref, but due to a bug, fails to prune the ref. When we fix
that bug, we would want to be doing the right thing here.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -468,7 +468,8 @@ __git_ps1 ()
 			fi
 		fi
 		if [ -n "${GIT_PS1_SHOWSTASHSTATE-}" ] &&
-		   [ -r "$g/refs/stash" ]; then
+		   git rev-parse --verify --quiet refs/stash >/dev/null
+		then
 			s="$"
 		fi