match: drop unnecessary adding of '' to set of dirs
This breaks some tests for "rootfilesin:" in a pattern matcher even
more, but that just shows how broken that case is.
Differential Revision: https://phab.mercurial-scm.org/D6406
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed