UI for specifying Git repo is very confusing

[talex5]

Problems:

1. If you run without arguments, it provides an in-memory-only store. This is only useful for debugging and should not be the default.
2. If you point it at a bare Git repository, it quietly adds /.git to the path, and thus creates a new empty repository inside the one you wanted to use. We should probably require a --auto-create option or similar to create a new repository, not change the path the user gives, and warn if they give a path with a .git subdirectory.

(discovered while testing with @jdjakub)

[talex5]

This requires a change to Irmin: @samoht will this be in the Irmin 1.0 API?

[samoht]

It could be added yes, is there an issue upstream?

[talex5]

I've added mirage/irmin#398 for this.