fast-export: ensure we export requested refs

fast-export: ensure we export requested refs

If file paths are specified to fast-export and a ref points to a commit
that does not touch any of the relevant paths, then that ref would
sometimes fail to be exported.  (This depends on whether any ancestors
of the commit which do touch the relevant paths would be exported with
that same ref name or a different ref name.)  To avoid this problem,
put *all* specified refs into extra_refs to start, and then as we export
each commit, remove the refname used in the 'commit $REFNAME' directive
from extra_refs.  Then, in handle_tags_and_duplicates() we know which
refs actually do need a manual reset directive in order to be included.

This means that we do need some special handling for excluded refs; e.g.
if someone runs
   git fast-export ^master master
then they've asked for master to be exported, but they have also asked
for the commit which master points to and all of its history to be
excluded.  That logically means ref deletion.  Previously, such refs
were just silently omitted from being exported despite having been
explicitly requested for export.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>