filter-branch: stash away ref map in a branch

filter-branch: stash away ref map in a branch

With "--state-branch=<branchname>" option, the mapping from old object names
and filtered ones in ./map/ directory is stashed away in the object database,
and the one from the previous run is read to populate the ./map/ directory,
allowing for incremental updates of large trees.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>