Rename linguist command to github-linguist

[lildude]

Description

As pointed out in #3843, when Linguist is installed, it creates a command of bin/linguist which clashes with Qt's bin/linguist.

@smola pointed it in #3843 (comment) that the Debian packages has addressed this by using the name github-linguist.

This PR changes the primary Linguist command from bin/linguist to bin/github-linguist and updates the tests and documentation to reflect this change.

This has been tested on GitHub.com with no adverse effects as it uses the bin/git-linguist command.

Checklist:

• I am adding new or changing current functionality
  • I have added or updated the tests for the new or changed functionality.

Fixes #3843

[lildude]

This is the commit that changed the name from bin/linguist to bin/github-linguist and is needed in order for the test_linguist_breakdown test to reflect the current state of the repo. This change is not essential, but nice to have but less likely to cause confusion later.

[lildude]

Not to self and anyone else who may be making a new Linguist release, this PR and #4222 introduce significant enough changes that a full version bump is called for on the next release.

/cc @kivikakk