Teach "apply --index-info" to handle rename patches

With v1.5.3.2~14 (apply --index-info: fall back to current index for
mode changes, 2007-09-17), git apply learned to stop worrying
about the lack of diff index line when a file already present in the
current index had no content change.

But it still worries too much: for rename patches, it is checking
that both the old and new filename are present in the current
index.  This makes no sense, since a file rename generally
involves creating a file there was none before.

So just check the old filename.

Noticed while trying to use “git rebase” with diff.renames = copies.

[jn: add tests]

Reported-by: David D. Kilzer <ddkilzer@kilzer.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -2979,8 +2979,7 @@ static void build_fake_ancestor(struct patch *list, const char *filename)
 		else if (get_sha1(patch->old_sha1_prefix, sha1))
 			/* git diff has no index line for mode/type changes */
 			if (!patch->lines_added && !patch->lines_deleted) {
-				if (get_current_sha1(patch->new_name, sha1) ||
-				    get_current_sha1(patch->old_name, sha1))
+				if (get_current_sha1(patch->old_name, sha1))
 					die("mode change for %s, which is not "
 						"in current HEAD", name);
 				sha1_ptr = sha1;

t/t4150-am.sh

@@ -116,6 +116,18 @@ test_expect_success setup '
 	git commit -m "added another file" &&
 
 	git format-patch --stdout master >lorem-move.patch &&
+
+	git checkout -b rename &&
+	git mv file renamed &&
+	git commit -m "renamed a file" &&
+
+	git format-patch -M --stdout lorem >rename.patch &&
+
+	git reset --soft lorem^ &&
+	git commit -m "renamed a file and added another" &&
+
+	git format-patch -M --stdout lorem^ >rename-add.patch &&
+
 	# reset time
 	unset test_tick &&
 	test_tick
@@ -246,8 +258,42 @@ test_expect_success 'am -3 falls back to 3-way merge' '
 	git diff --exit-code lorem
 '
 
+test_expect_success 'am can rename a file' '
+	grep "^rename from" rename.patch &&
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout lorem^0 &&
+	git am rename.patch &&
+	! test -d .git/rebase-apply &&
+	git update-index --refresh &&
+	git diff --exit-code rename
+'
+
+test_expect_success 'am -3 can rename a file' '
+	grep "^rename from" rename.patch &&
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout lorem^0 &&
+	git am -3 rename.patch &&
+	! test -d .git/rebase-apply &&
+	git update-index --refresh &&
+	git diff --exit-code rename
+'
+
+test_expect_success 'am -3 can rename a file after falling back to 3-way merge' '
+	grep "^rename from" rename-add.patch &&
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout lorem^0 &&
+	git am -3 rename-add.patch &&
+	! test -d .git/rebase-apply &&
+	git update-index --refresh &&
+	git diff --exit-code rename
+'
+
 test_expect_success 'am -3 -q is quiet' '
 	rm -fr .git/rebase-apply &&
+	git checkout -f lorem2 &&
 	git reset master2 --hard &&
 	sed -n -e "3,\$p" msg >file &&
 	head -n 9 msg >>file &&