fsmonitor: refactor untracked-cache invalidation

fsmonitor: refactor untracked-cache invalidation

Update fsmonitor_refresh_callback() to use the new
untracked_cache_invalidate_trimmed_path() to invalidate
the cache using the observed pathname without needing to
modify the caller's buffer.

Previously, we modified the caller's buffer when the observed pathname
contained a trailing slash (and did not restore it).  This wasn't a
problem for the single use-case caller, but felt dirty nontheless.  In
a later commit we will want to invalidate case-corrected versions of
the pathname (using possibly borrowed pathnames from the name-hash or
dir-name-hash) and we may not want to keep the tradition of altering
the passed-in pathname.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>