Infrastructure to write revisions in fast-export format

Infrastructure to write revisions in fast-export format

repo_tree maintains the exporter's state and provides a facility to to
call fast_export, which writes objects to stdout suitable for
consumption by fast-import.

The exported functions roughly correspond to Subversion FS operations.

 . repo_add, repo_modify, repo_copy, repo_replace, and repo_delete
   update the current commit, based roughly on the corresponding
   Subversion FS operation.

 . repo_commit calls out to fast_export to write the current commit to
   the fast-import stream in stdout.

 . repo_diff is used by the fast_export module to write the changes
   for a commit.

 . repo_reset erases the exporter's state, so valgrind can be happy.

[rr: squelched compiler warnings]
[jn: removed support for maintaining state on-disk, though we may
want to add it back later]

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>