Address issue #4675: fix Git.check_version()

This was suggested by @xavfernandez in the review comments.

* Add failing test for issue #4507.

* Add looks_like_hash() with tests.

* Add news file.

* Address issue #4507 by using looks_like_hash().

* Tweak warning text.

* Fix test after rebasing.

* Remove extra line.

Strip usage in the documentation

Stop depending on platform.libc_ver()

This removes a useless empty line in the output.

Address issue #1130 (GIT_DIR and GIT_WORK_TREE).

* .travis.tml: Enable fast_finish

https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

* https://pip.pypa.io/en/latest/development/#adding-a-news-entry

* Autocomplete installed dists in `pip show ...`.

* 🎨 Assign to a variable to satisfy linters

* fix test not using temporary directory
* avoid creating a `cache_dir` directory in the source tree
* drop `data` fixture from tests that don't use it

* tests: fix pytest warning

* Add the RevOptions class, and test.

* Start using the RevOptions class.

* Add news file.

* Update for mypy.

* Fix test after rebasing.

* Address @xavfernandez's review comments.

Empirically, pip supports pip [-e] hg+file:///path/to/repo

* Add failing test

* Don't assume master branch is default

* Don't expect output to stderr

* Use the 'short ref' instead of the 'full ref'

* Improve repeated command line options sentence

* Add trivial news entry

* Update user_guide.rst

* Shorten sentence

* Add documentation on how to call pip from user code
* Document the lack of a Python API for pip

Fix Tests by depending on testpypi

Upgrade our Vendored Dependencies

Remove a redundant import

Get rid of successfully_installed attr

The linters missed it. Funny.