The code tries to collapse identical leading components
between the prefix and the path. So if we're in "dir1", the
path "dir1/file" should become just "file". However, we were
ending up with "../dir1/file". The included test expected
the wrong output.

The "len" parameter to quote_path can be negative to mean
"this is a NUL terminated string".  Simply count it so that
the loop can rely on it being the length of the path.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>