The check was added in 31abcae33b4f (dirstate: do not ignore current
directory '.' (issue 1078), 2008-04-05) to fix issue1078. Funnily
enough, comment #2 on that issue mentions using '' instead of '.' to
represent the root directory, just like my previous patch did.
test-hgignore.t fails with this patch without the previous patch.