detached-stash: work around git rev-parse failure to detect bad log refs

This commit is required because git rev-parse in 1.7.2 does not correctly
indicate invalid log references using a non-zero status code.

We use a proxy for the condition (non-empty error output) as
a substitute. This commit can be reverted when, and if, rev-parse
is fixed to indicate invalid log references with a status code.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -328,6 +328,15 @@ parse_flags_and_rev()
 	test "$ref_stash" = "$(git rev-parse --symbolic-full-name "${REV%@*}")" &&
 	IS_STASH_REF=t
 
+	if test "${REV}" != "${REV%{*\}}"
+	then
+		# maintainers: it would be better if git rev-parse indicated
+		# this condition with a non-zero status code but as of 1.7.2.1 it
+		# it did not. So, we use non-empty stderr output as a proxy for the
+		# condition of interest.
+		test -z "$(git rev-parse "$REV" 2>&1 >/dev/null)" || die "$REV does not exist in the stash log"
+	fi
+
 }
 
 is_stash_like()