prefix_path(): unconditionally free results in the callers

prefix_path(): unconditionally free results in the callers

As of d089ebaa (setup: sanitize absolute and funny paths in
get_pathspec(), 2008-01-28), prefix_path() always returns a
newly allocated string, so callers should free its result.

Additionally, drop the const from variables to which the result of
the prefix_path() is assigned, so they can be free()'d without
having to cast-away the constness.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>