fast-import: add support for new 'alias' command

fast-import: add support for new 'alias' command

fast-export and fast-import have nice --import-marks flags which allow
for incremental migrations.  However, if there is a mark in
fast-export's file of marks without a corresponding mark in the one for
fast-import, then we run the risk that fast-export tries to send new
objects relative to the mark it knows which fast-import does not,
causing fast-import to fail.

This arises in practice when there is a filter of some sort running
between the fast-export and fast-import processes which prunes some
commits programmatically.  Provide such a filter with the ability to
alias pruned commits to their most recent non-pruned ancestor.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>