The existing checkpoint command is very useful to force fast-import
to dump the branches out to disk so that standard Git tools can
access them and the objects they refer to.  However there was not a
way to know when fast-import had finished executing the checkpoint
and it was safe to read those refs.

The progress command can be used to make fast-import output any
message of the frontend's choosing to standard out.  The frontend
can scan for these messages using select() or poll() to monitor a
pipe connected to the standard output of fast-import.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>