dir: create untracked_cache_invalidate_trimmed_path()

dir: create untracked_cache_invalidate_trimmed_path()

Create a wrapper function for untracked_cache_invalidate_path()
that silently trims a trailing slash, if present, before calling
the wrapped function.

The untracked cache expects to be called with a pathname that
does not contain a trailing slash.  This can make it inconvenient
for callers that have a directory path.  Lets hide this complexity.

This will be used by a later commit in the FSMonitor code which
may receive directory pathnames from an FSEvent.

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