mv: allow renaming to fix case on case insensitive filesystems

"git mv hello.txt Hello.txt" on a case insensitive filesystem
always triggers "destination already exists" error, because these
two names refer to the same path from the filesystem's point of
view, and requires the user to give "--force" when correcting the
case of the path recorded in the index and in the next commit.

Detect this case and allow it without requiring "--force".
Signed-off-by: NDavid Turner <dturner@twitter.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/mv.c

@@ -202,7 +202,8 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 			}
 		} else if (cache_name_pos(src, length) < 0)
 			bad = _("not under version control");
-		else if (lstat(dst, &st) == 0) {
+		else if (lstat(dst, &st) == 0 &&
+			 (!ignore_case || strcasecmp(src, dst))) {
 			bad = _("destination exists");
 			if (force) {
 				/*

t/t6039-merge-ignorecase.sh

@@ -35,7 +35,7 @@ test_expect_success 'merge with case-changing rename on both sides' '
 	git reset --hard baseline &&
 	git branch -D with-camel &&
 	git checkout -b with-camel &&
-	git mv --force TestCase testcase &&
+	git mv TestCase testcase &&
 	git commit -m "recase on branch" &&
 	>foo &&
 	git add foo &&