overlayworkingctx: remove misleading trailing slash from directory pattern
The paths passed into the matcher are normalized (this applies to
include patterns and regular patterns, and to both glob kind and path
kind), so the regex for input "foo/" ended up being "foo(?:/|$)".
Once we have a (recursive) pattern kind only for directories, we could
switch to that here and remove the "mfiles[0] == path" check. Until
then, let's at least make it not misleading.
Differential Revision: https://phab.mercurial-scm.org/D6224