bash prompt: use bash builtins to check stash state

When the environment variable $GIT_PS1_SHOWSTASHSTATE is set
__git_ps1() checks the presence of stashes by running 'git rev-parse
--verify refs/stash'.  This command not only checks that the
'refs/stash' ref exists but also, well, verifies that it's a valid
ref.

However, we don't need to be that thorough for the bash prompt.  We
can omit that verification and only check whether 'refs/stash' exists
or not.  Since 'git pack-refs' never packs 'refs/stash', it's a matter
of checking the existence of a ref file.  Perform this check using
only bash builtins to spare the overhead of fork()+exec()ing a git
process.

Also run 'git pack-refs --all' in the corresponding test to document
that the prompt script depends on 'git pack-refs' not packing
'refs/stash' and to catch possible breakages should this behavior ever
change.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>

contrib/completion/git-prompt.sh

@@ -435,8 +435,9 @@ __git_ps1 ()
 				i="#"
 			fi
 		fi
-		if [ -n "${GIT_PS1_SHOWSTASHSTATE-}" ]; then
+		if [ -n "${GIT_PS1_SHOWSTASHSTATE-}" ] &&
-			git rev-parse --verify refs/stash >/dev/null 2>&1 && s="$"
+		   [ -r "$g/refs/stash" ]; then
+			s="$"
 		fi
 
 		if [ -n "${GIT_PS1_SHOWUNTRACKEDFILES-}" ] &&

t/t9903-bash-prompt.sh

@@ -328,6 +328,7 @@ test_expect_success 'prompt - stash status indicator - stash' '
 	echo 2 >file &&
 	git stash &&
 	test_when_finished "git stash drop" &&
+	git pack-refs --all &&
 	(
 		GIT_PS1_SHOWSTASHSTATE=y &&
 		__git_ps1 >"$actual"