dvcignore: don't look forward (#5098)
dvcignore: don't look forward (#5098)

Prevents us from duplicating the work by walking into directories
searching for subrepos. Saves around ~1sec (5.8 -> 4.8) in
`dvc metrics diff` in a big git-only repo.

Related to https://github.com/iterative/dvc/pull/4284#issuecomment-664033700
dmpetrov/dataversioncontrol,dmpetrov/dataversioncontrol,efiop/dvc,efiop/dvc