ref-filter: simplify lstrip_ref_components() memory handling

ref-filter: simplify lstrip_ref_components() memory handling

We're walking forward in the string, skipping path components from
left-to-right. So when we've stripped as much as we want, the pointer we
have is a complete NUL-terminated string and we can just return it
(after duplicating it, of course). So there is no need for a temporary
allocated string.

But we do make an extra temporary copy due to f0062d3b74 (ref-filter:
free item->value and item->value->s, 2018-10-18). This is probably from
cargo-culting the technique used in rstrip_ref_components(), which
_does_ need a separate string (since it is stripping from the end and
ties off the temporary string with a NUL).

Let's drop the extra allocation. This is slightly more efficient, but
more importantly makes the code much simpler.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>