rev-list: allow filtering of provided items

rev-list: allow filtering of provided items

When providing an object filter, it is currently impossible to also
filter provided items. E.g. when executing `git rev-list HEAD` , the
commit this reference points to will be treated as user-provided and is
thus excluded from the filtering mechanism. This makes it harder than
necessary to properly use the new `--filter=object:type` filter given
that even if the user wants to only see blobs, he'll still see commits
of provided references.

Improve this by introducing a new `--filter-provided-objects` option
to the git-rev-parse(1) command. If given, then all user-provided
references will be subject to filtering.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>