fast-import: checkpoint: dump branches/tags/marks even if object_count==0

The checkpoint command cycles packfiles if object_count != 0, a sensible
test or there would be no pack files to write. Since 820b931012, the
command also dumps branches, tags and marks, but still conditionally.
However, it is possible for a command stream to modify refs or create
marks without creating any new objects.

For example, reset a branch (and keep fast-import running):

	$ git fast-import
	reset refs/heads/master
	from refs/heads/master^

	checkpoint

but refs/heads/master remains unchanged.

Other example: a commit command that re-creates an object that already
exists in the object database.

The man page also states that checkpoint "updates the refs" and that
"placing a progress command immediately after a checkpoint will inform
the reader when the checkpoint has been completed and it can safely
access the refs that fast-import updated". This wasn't always true
without this patch.

This fix unconditionally calls dump_{branches,tags,marks}() for all
checkpoint commands. dump_branches() and dump_tags() are cheap to call
in the case of a no-op.

Add tests to t9300 that observe the (non-packfiles) effects of
checkpoint.

Signed-off-by: Eric Rannaud <e@nanocritical.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -3188,10 +3188,10 @@ static void checkpoint(void)
 	checkpoint_requested = 0;
 	if (object_count) {
 		cycle_packfile();
-		dump_branches();
-		dump_tags();
-		dump_marks();
 	}
+	dump_branches();
+	dump_tags();
+	dump_marks();
 }
 
 static void parse_checkpoint(void)