hooks: checkout: fix incorrect dvc repo detection

When running from non-root directory `git ls-files .dvc` will treat it
as a relative path to the current location, so if you run `git checkout`
from a directory that is not a git/dvc root, our hook will think that
it is being run in a non-dvc repo and will do nothing. To fix that, need
to use `--full-name` flag.

dvc/scm/git/__init__.py

@@ -250,7 +250,7 @@ class Git(Base):
     def _install_hook(self, name, cmd):
         command = (
             '[ "$3" = "0" ]'
-            ' || [ -z "$(git ls-files .dvc)" ]'
+            ' || [ -z "$(git ls-files --full-name .dvc)" ]'
             " || exec dvc {}".format(cmd)
         )