apply --index-info: fall back to current index for mode changes

"git diff" does not record index lines for pure mode changes (i.e. no
lines changed).  Therefore, apply --index-info would call out a bogus
error.

Instead, fall back to reading the info from the current index.

Incidentally, this fixes an error where git-rebase would not rebase a
commit including a pure mode change, and changes requiring a threeway
merge.

Noticed and later tested by Chris Shoemaker.

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

builtin-apply.c

@@ -2227,6 +2227,20 @@ static int check_patch_list(struct patch *patch)
 	return err;
 }
 
+/* This function tries to read the sha1 from the current index */
+static int get_current_sha1(const char *path, unsigned char *sha1)
+{
+	int pos;
+
+	if (read_cache() < 0)
+		return -1;
+	pos = cache_name_pos(path, strlen(path));
+	if (pos < 0)
+		return -1;
+	hashcpy(sha1, active_cache[pos]->sha1);
+	return 0;
+}
+
 static void show_index_list(struct patch *list)
 {
 	struct patch *patch;
@@ -2243,8 +2257,16 @@ static void show_index_list(struct patch *list)
 		if (0 < patch->is_new)
 			sha1_ptr = null_sha1;
 		else if (get_sha1(patch->old_sha1_prefix, sha1))
-			die("sha1 information is lacking or useless (%s).",
-			    name);
+			/* 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))
+					die("mode change for %s, which is not "
+						"in current HEAD", name);
+				sha1_ptr = sha1;
+			} else
+				die("sha1 information is lacking or useless "
+					"(%s).", name);
 		else
 			sha1_ptr = sha1;
 

t/t3400-rebase.sh

@@ -68,4 +68,19 @@ test_expect_success \
      test 3 = $(git rev-list master.. | wc -l)
 '
 
+test_expect_success 'rebase a single mode change' '
+     git checkout master &&
+     echo 1 > X &&
+     git add X &&
+     test_tick &&
+     git commit -m prepare &&
+     git checkout -b modechange HEAD^ &&
+     echo 1 > X &&
+     git add X &&
+     chmod a+x A &&
+     test_tick &&
+     git commit -m modechange A X &&
+     GIT_TRACE=1 git rebase master
+'
+
 test_done