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 Plan:
```
$ git clone ssh://local@localvault.phacility.com/source/quack.notgit/
Cloning into 'quack.notgit'...
phabricator-ssh-exec: This repository ("quack.notgit") is not a Git repository. Use "hg" to interact with this repository.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
```
```
$ hg clone ssh://local@localvault.phacility.com/source/phabx
remote: phabricator-ssh-exec: This repository ("phabx") is not a Mercurial repository. Use "git" to interact with this repository.
abort: no suitable response from remote hg!
```
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11938
Differential Revision: https://secure.phabricator.com/D16976