backfill: accept revision arguments

backfill: accept revision arguments

The existing implementation of 'git backfill' only includes downloading
missing blobs reachable from HEAD. Advanced uses may desire more general
commit limiting options, such as '--all' for all references, specifying a
commit range via negative references, or specifying a recency of use such as
with '--since=<date>'.

All of these options are available if we use setup_revisions() to parse the
unknown arguments with the revision machinery. This opens up a large number
of possibilities, only a small set of which are tested here.

For documentation, we avoid duplicating the option documentation and instead
link to the documentation of 'git rev-list'.

Note that these arguments currently allow specifying a pathspec, which
modifies the commit history checks but does not limit the paths used in the
backfill logic. This will be updated in a future change.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>