Teach git-stash to "apply --index"

When given this subcommand, git-stash will try to merge the stashed
index into the current one. Only trivial merges are possible, since
we have no index for the index ;-) If a trivial merge is not possible,
git-stash will bail out with a hint to skip the --index option.

For good measure, finally include a test case.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3903-stash.sh

@@ -0,0 +1,69 @@
+#!/bin/sh
+#
+# Copyright (c) 2007 Johannes E Schindelin
+#
+
+test_description='Test git-stash'
+
+. ./test-lib.sh
+
+test_expect_success 'stash some dirty working directory' '
+	echo 1 > file &&
+	git add file &&
+	test_tick &&
+	git commit -m initial &&
+	echo 2 > file &&
+	git add file &&
+	echo 3 > file &&
+	test_tick &&
+	git stash &&
+	git diff-files --quiet &&
+	git diff-index --cached --quiet HEAD
+'
+
+cat > expect << EOF
+diff --git a/file b/file
+index 0cfbf08..00750ed 100644
+--- a/file
++++ b/file
+@@ -1 +1 @@
+-2
++3
+EOF
+
+test_expect_success 'parents of stash' '
+	test $(git rev-parse stash^) = $(git rev-parse HEAD) &&
+	git diff stash^2..stash > output &&
+	diff -u output expect
+'
+
+test_expect_success 'apply needs clean working directory' '
+	echo 4 > other-file &&
+	git add other-file &&
+	echo 5 > other-file
+	! git stash apply
+'
+
+test_expect_success 'apply stashed changes' '
+	git add other-file &&
+	test_tick &&
+	git commit -m other-file &&
+	git stash apply &&
+	test 3 = $(cat file) &&
+	test 1 = $(git show :file) &&
+	test 1 = $(git show HEAD:file)
+'
+
+test_expect_success 'apply stashed changes (including index)' '
+	git reset --hard HEAD^ &&
+	echo 6 > other-file &&
+	git add other-file &&
+	test_tick &&
+	git commit -m other-file &&
+	git stash apply --index &&
+	test 3 = $(cat file) &&
+	test 2 = $(git show :file) &&
+	test 1 = $(git show HEAD:file)
+'
+
+test_done