Return integers, not strings from get_commit_dates()

Returning strings broke bup fuse (surprising that it worked
elsewhere), and since we have no fuse tests, it wasn't immediately
apparent.

So fix the bug and add some initial (trivial) fuse tests so that at
least this particular problem doesn't happen again.

To cleanup after the fuse tests, detect fuse mounts in
t/cleanup-mounts-under, and try to unmount them.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>