pip freeze doesn't show correct entry for mercurial packages that use subdirectories

[TonyBeswick]

Environment

• pip version: 19.2.3
• Python version: 2.7.15 & 3.7.3
• OS: win10 & Ubuntu 18.04.3 LTS

Description
When a package is installed from a Mercurial (hg) repo and it uses either:

• a setup.py file that isn't located in the root directory of the repo, or
• a package directory that isn't located in the root directory of the repo (e.g. uses a src directory)

Then pip freeze will output: # Editable install with no version control for that package and omit the reference to version control.

#3258 added the ability for pip freeze and pip list -e to work correctly when either setup.py or the source code is not in the root directory of a Git repository. This implementation was Git specific, the problem solved by #3258 still exists for Mercurial repositories.

How to Reproduce

1. pip install a mercurial repository that has:
   • a setup.py file that isn't located in the root directory of the repo, or
   • a package directory that isn't located in the root directory of the repo (e.g. uses a src directory)
2. run pip freeze and look for # Editable install with no version control

Its difficult to find such a repository publicly, so here is some test code that can be added to tests/functional/test_freeze.py

@need_mercurial
def test_freeze_mercurial_clone_srcdir(script, tmpdir):
    """
    Test freezing a Mercurial clone where setup.py is in a subdirectory
    relative to the repo root and the source code is in a subdirectory
    relative to setup.py.
    """
    # Returns path to a generated package called "version_pkg"
    pkg_version = _create_test_package_with_srcdir(script, vcs='hg')
 
    result = script.run(
        'hg', 'clone', pkg_version, 'pip-test-package',
        expect_stderr=True,
    )
    repo_dir = script.scratch_path / 'pip-test-package'
    result = script.run(
        'python', 'setup.py', 'develop',
        cwd=repo_dir / 'subdir',
        expect_stderr=True,
    )
    result = script.pip('freeze', expect_stderr=True)
    expected = textwrap.dedent(
        """
            ...-e hg+...#egg=version_pkg&subdirectory=subdir
            ...
        """
    ).strip()
    _check_output(result.stdout, expected)
 
    result = script.pip(
        'freeze', '-f', '%s#egg=pip_test_package' % repo_dir,
        expect_stderr=True,
    )
    expected = textwrap.dedent(
        """
            -f %(repo)s#egg=pip_test_package...
            -e hg+...#egg=version_pkg&subdirectory=subdir
            ...
        """ % {'repo': repo_dir},
    ).strip()
    _check_output(result.stdout, expected)