Dump all refs and marks during a checkpoint in fast-import.

Dump all refs and marks during a checkpoint in fast-import.

If the frontend asks us to checkpoint (via the explicit checkpoint
command) its probably because they are afraid the current import
will crash/fail/whatever and want to make sure they can pickup from
the last checkpoint.  To do that sort of recovery, we will need the
current tip of every branch and tag available at the next startup.

Signed-off-by: Shawn O. Pearce <redacted>