* mu-proc: Don't rename when moving inside a Maildir

`mbsync` expects messages moving inside a Maildir (from `new` to `cur`, for example) to mantain its basename.

This change tries to fix issue #321. It might be more sensible than the change proposed there. I can't see this negatively affecting other users, as it is my impression that the whole `mu4e-change-filenames-when-moving` was introduced to satisfy `mbsync`.

What do you think?