Add a small patch-mode testing library

The tests for {reset,commit,stash} -p will frequently have to set both
worktree and index states to known values, and verify that the outcome
(again both worktree and index) are what was expected.

Add a small helper library that lets us do these tasks more easily.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-patch-mode.sh

@@ -0,0 +1,36 @@
+. ./test-lib.sh
+
+set_state () {
+	echo "$3" > "$1" &&
+	git add "$1" &&
+	echo "$2" > "$1"
+}
+
+save_state () {
+	noslash="$(echo "$1" | tr / _)" &&
+	cat "$1" > _worktree_"$noslash" &&
+	git show :"$1" > _index_"$noslash"
+}
+
+set_and_save_state () {
+	set_state "$@" &&
+	save_state "$1"
+}
+
+verify_state () {
+	test "$(cat "$1")" = "$2" &&
+	test "$(git show :"$1")" = "$3"
+}
+
+verify_saved_state () {
+	noslash="$(echo "$1" | tr / _)" &&
+	verify_state "$1" "$(cat _worktree_"$noslash")" "$(cat _index_"$noslash")"
+}
+
+save_head () {
+	git rev-parse HEAD > _head
+}
+
+verify_saved_head () {
+	test "$(cat _head)" = "$(git rev-parse HEAD)"
+}