stash drops the stash even if creating the branch fails because it already exists

This bug was disovered by someone on IRC when he tried to

    $ git stash branch <branch> <stash>

while <branch> already existed. In that case the stash is dropped even
though it isn't applied on any branch, so the stash is effectively lost.

Signed-off-by: Tomas Carnecky <tom@dbservice.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3903-stash.sh

@@ -545,4 +545,15 @@ test_expect_success 'invalid ref of the form stash@{n}, n >= N' '
 	git stash drop
 '
 
+test_expect_failure 'stash branch should not drop the stash if the branch exists' '
+	git stash clear &&
+	echo foo >file &&
+	git add file &&
+	git commit -m initial &&
+	echo bar >file &&
+	git stash &&
+	test_must_fail git stash branch master stash@{0} &&
+	git rev-parse stash@{0} --
+'
+
 test_done