fast-export: Allow pruned-references in mark file

fast-export: Allow pruned-references in mark file

fast-export can fail because of some pruned-reference when importing a
mark file.

The problem happens in the following scenario:

    $ git fast-export --export-marks=MARKS master
    (rewrite master)
    $ git prune
    $ git fast-export --import-marks=MARKS master

This might fail if some references have been removed by prune
because some marks will refer to no longer existing commits.
git-fast-export will not need these objects anyway as they were no
longer reachable.

We still need to update last_numid so we don't change the mapping
between marks and objects for remote-helpers.
Unfortunately, the mark file should not be rewritten without lost marks
if no new objects has been exported, as we could lose track of the last
last_numid.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Reviewed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>