DWIM .git repository discovery

Hi,

I often find myself attempting to examine another repository, especially in projects that are closely related but put in different git repos. It's usually just a diff or log command

git log --patch ../path/to/another/repo/path/to/file.c

cd'ing out is nuisance for one-shot commands, even setting --git-dir is because i'd need to repeat the path. I think when we detect paths outside repository, we could try to discover new repository that contain those paths, the adjust $GIT_DIR internally to the new repository.

We can't do that now because .git repository and "path outside repository" check occur in two separate stages, the latter after .git is discovered and .git can't be moved once discovered. We might be able to move "path outside repo" check up to discovery phase. But that could be big change.

Does anybody want such a feature, or it's just my itch? It's probably not worth doing because few people need it.