Teach fast-import to print the id of each imported commit

Teach fast-import to print the id of each imported commit

For the svn importer, it would be useful to build a map from
subversion revision numbers to git commits as the import takes place.
This is particularly relevant because the subversion api sometimes
represents as "copy this directory from this revision", and the
importer needs to be able to access the corresponding trees.  So
(optionally) print each commit id when the corresponding object is
written.

Unfortunately when each commit object is written, it is not yet
accessible to the caller until a checkpoint has finished.  A later
patch will teach fast-import to directly pass on the relevant data on
request, using the same channel.

Cc: Shawn O. Pearce <spearce@spearce.org>
Cc: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>