R1:faf983614cd5

Improve error messages for running `git clone` against a Mercurial repository

Summary:
Fixes T11938.

Note that there's a subcase here: if you `hg clone` or `svn checkout` a short `/source/` URI that ends in `.git`, we miss the lookup and don't get this far, so you still get a generic error message.

Hopefully it is clear enough on its own that `proto://.../blah.git` is, in fact, a Git repository, since it says ".git" at the end.

If that doesn't prove to be true, we can be more surgical about this.

Test…