Teach fast-import to import subtrees named by tree id

Teach fast-import to import subtrees named by tree id

To simulate the svn cp command, it would be very useful to be
replace an arbitrary file in the current revision by an
arbitrary directory from a previous one.  Modify the filemodify
command to allow that:

 M 040000 <tree id> pathname

This would be most useful in combination with a facility to
print the commit ids for new revisions as they are written.

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