Add support to provide ".git" folder location

[datosh]

Currently go-git only provides a public API to get the location of the git root folder via:

tree, err := repository.Worktree()
tree.Filesystem.Root()

Getting from the repository root directory to the .git directory is not trivial since .git can be both a folder or a file referencing a folder.
This logic is already implemented in go-git (https://github.com/go-git/go-git/blob/master/repository.go#L332-L417 )but not available as public API.

My suggestion would be to extend the Repository API to provide a DotGit() string function that returns the folder to the .git folder of the current repository.

[pjbgf]

@datosh this could be a new Interface that the supporting Storage.Storers could implement. But I am assuming this would only be needed for filesystem though.

Note that if you are using BoundOS it may be slightly easier to have a direct translation of the path.