verify_dotfile(): reject .git case-insensitively

We do not allow ".git" to enter into the index as a path
component, because checking out the result to the working
tree may causes confusion for subsequent git commands.
However, on case-insensitive file systems, ".Git" or ".GIT"
is the same. We should catch and prevent those, too.

Note that technically we could allow this for repos on
case-sensitive filesystems. But there's not much point. It's
unlikely that anybody cares, and it creates a repository
that is unexpectedly non-portable to other systems.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -759,9 +759,10 @@ static int verify_dotfile(const char *rest)
 	 * shares the path end test with the ".." case.
 	 */
 	case 'g':
-		if (rest[1] != 'i')
+	case 'G':
+		if (rest[1] != 'i' && rest[1] != 'I')
 			break;
-		if (rest[2] != 't')
+		if (rest[2] != 't' && rest[2] != 'T')
 			break;
 		rest += 2;
 	/* fallthrough */

t/t1014-read-tree-confusing.sh

@@ -27,6 +27,7 @@ done <<-\EOF
 .
 ..
 .git
+.GIT
 EOF
 
 test_done