Fix behaviour of commit. It's now a fatal error if the option to --logfile isn't readable. Ditto if both --message and --logfile are specified.

Unify display of dates. We now use one function, commands.datestr, to display dates, instead of calling time.asctime in inconsistent ways.

Print commit time/date from perspective of committer. Previously, it was printed in the time zone of the reader, but with the committer's time zone offset. It is now consistent.