detached-stash: simplify git stash branch

This patch teaches git stash branch to tolerate stash-like arguments.

In particular, a stash is only required if an argument isn't specified
and the stash is only dropped if a stash entry reference was
specified or implied.

The implementation has been simplified by taking advantage of
assert_stash_like() and the variables established by
parse_flags_and_rev().

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

git-stash.sh

@@ -441,20 +441,17 @@ drop_stash () {
 }
 
 apply_to_branch () {
-	have_stash || die 'Nothing to apply'
-
 	test -n "$1" || die 'No branch name specified'
 	branch=$1
+	shift 1
 
-	if test -z "$2"
-	then
-		set x "$ref_stash@{0}"
-	fi
-	stash=$2
+	set -- --index "$@"
+	assert_stash_like "$@"
+
+	git checkout -b $branch $REV^ &&
+	apply_stash "$@"
 
-	git checkout -b $branch $stash^ &&
-	apply_stash --index $stash &&
-	drop_stash $stash
+	test -z "$IS_STASH_REF" || drop_stash "$@"
 }
 
 PARSE_CACHE='--not-parsed'