a remote url from what git uses to what pip accepts.

There are 3 legal forms **url** may take:

    1. A fully qualified url: ssh://git@example.com/foo/bar.git
    2. A local project.git folder: /path/to/bare/repository.git
    3. SCP shorthand for form 1: git@example.com:foo/bar.git

Form 1 is output as-is. Form 2 must be converted to URI and form 3 must
be converted to form 1.

See the corresponding test test_git_remote_url_to_pip() for examples of
sample inputs/outputs.
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