fast-import: do not truncate exported marks file

fast-import: do not truncate exported marks file

Certain lines of the marks file might be corrupted (or the objects
missing due to a garbage collection), but that's no reason to truncate
the file and essentially destroy the rest of it.

Ideally missing objects should not cause a crash, we could just skip
them, but that's another patch.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>