The sleep was to force the directory's mtime to advance between the
previous notmuch new and the subsequent rm;notmuch new.
The current convention is to use the existing increment_mtime function
for this purpose, (which avoids the test suite being slowed down by
calls to sleep).
Thanks to Austin Clements for noticing this undesired sleep.
Test for bug. Current stemming support for notmuch adds extra terms
to the DB which aren't removed when the file renames are detected.
When folder tags are added to a message, Xapian terms for both XFOLDER
and ZXFOLDER are generated. When one of the filenames are
renamed/removed, only the XFOLDER tags are removed, leaving it possible
for a match on a folder: tag that was previously but is no longer a
match in the maildir.
