Accept branch name as argument

[bval]

Modify the github-linguist CLI to accept a new, optional argument specifying a git revision other than HEAD to analyze.

Description

While working with a GitHub customer it came to my attention that linguist as a command line tool will only analyze HEAD in the current repo, which is typically the default branch in a newly cloned repo. I have added an optional command line flag to accept any gitrevisions(1) compatible ref so that other branches, tags, or individual SHAs can be analyzed provided they are reachable.

This does not regress any current functionality, and I also rescue the potential exception should the user provide a revision that is not reachable.

I have investigated adding a test but there appear to be none in the current suite related to the CLI at all, and this change should be safe to introduce without one.

This makes the tool useful for example in CI pipelines where the languages present may be an input to a later step, where certain build tools, linters, or scanning tools may be needed based on the results.

Checklist:

• I am associating a language with a new file extension.

  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3AFOOBAR+KEYWORDS+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am adding a new language.

  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3AFOOBAR+KEYWORDS+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a syntax highlighting grammar: [URL to grammar repo]
  • I have updated the heuristics to distinguish my language from others using the same extension.

• I am fixing a misclassified language

  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am changing the source of a syntax highlighting grammar

  • Old: [URL to grammar repo]
  • New: [URL to grammar repo]

• I am updating a grammar submodule

• I am adding new or changing current functionality

  • I have added or updated the tests for the new or changed functionality.

• I am changing the color associated with a language

  • I have obtained agreement from the wider language community on this color change.
    • [URL to public discussion]
    • [Optional: URL to official branding guidelines for the language]

[rewinfrey]

👋 @bval, thanks for adding this functionality. This patch looks good to me!

We'll need @lildude for a final sign off.

[lildude]

LGTM. Sorry about the delay.