notmuch new: Don't ignore files with mtime of 0.

notmuch new: Don't ignore files with mtime of 0.

I recently discovered that mb2md has the annoying bug of creating
files with mtime of 0, and notmuch then promptly ignored them,
(thinking that its timestamps initialized to 0 were just as new).

We fix notmuch to not exclude messages based on a database timestamp
of 0.