notmuch-new: Test if directory looks like Maildir before skipping tmp.

'notmuch new' skips directory entries with the name 'tmp'. This is to
prevent notmuch from processing possibly incomplete Maildir messages
stored in that directory.

This patch attempts to refine the feature. If "tmp" entry is found,
it first checks if the containing directory looks like a Maildir
directory. This is done by searching for other common Maildir
subdirectories. If they exist and if the entry "tmp" is a directory
then it is skipped.

Files and subdirectories with the name "tmp" that do not look like
Maildir will still be processed by 'notmuch new'.

Signed-off-by: Jan Janak <jan@ryngle.com>