Improve error messages from 'git stash show'

The previous error message "fatal: Needed a single revision" is not
very informative.

Signed-off-by: Gustaf Hendeby <hendeby@isy.liu.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: hendeby

git-stash.sh

@@ -210,14 +210,18 @@ list_stash () {
 }
 
 show_stash () {
+	have_stash || die 'No stash found'
+
 	flags=$(git rev-parse --no-revs --flags "$@")
 	if test -z "$flags"
 	then
 		flags=--stat
 	fi
 
-	w_commit=$(git rev-parse --verify --default $ref_stash "$@") &&
-	b_commit=$(git rev-parse --verify "$w_commit^") &&
+	w_commit=$(git rev-parse --quiet --verify --default $ref_stash "$@") &&
+	b_commit=$(git rev-parse --quiet --verify "$w_commit^") ||
+		die "'$*' is not a stash"
+
 	git diff $flags $b_commit $w_commit
 }