This defines backup log file format and adds basic support for writing
new entries to backup log files. The format is the same as reflog
except that "message" field becomes "path".

Similar to reflog, updating is done by appending to the end of the file
instead of creating a branch new file and do an atomic rename. If the
backup log file gets large, regenerating whole file could take longer,
and we should keep backup log overhead to minimum since it will be
called by a bunch of commands later.