verify_path: consider dos drive prefix

If someone manage to create a repo with a 'C:' entry in the
root-tree, files can be written outside of the working-dir. This
opens up a can-of-worms of exploits.

Fix it by explicitly checking for a dos drive prefix when verifying
a paht. While we're at it, make sure that paths beginning with '\' is
considered absolute as well.

Noticed-by: Theo Niessink <theo@taletn.com>
Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -774,11 +774,14 @@ int verify_path(const char *path)
 {
 	char c;
 
+	if (has_dos_drive_prefix(path))
+		return 0;
+
 	goto inside;
 	for (;;) {
 		if (!c)
 			return 1;
-		if (c == '/') {
+		if (is_dir_sep(c)) {
 inside:
 			c = *path++;
 			switch (c) {