Implements build-in support for linting the commits of git sub modules, which currently doesn't work because sub modules don't have their own .git folder.

Description

I updated the read module to determine if it is running from a git sub module or not and find the COMMIT_EDITMSG accordingly. It still uses edit argument if present, otherwise it looks at .git and checks whether it is a directory (if it is, look for the COMMIT_EDITMSG as before) or a file, in which case it reads the file to figure out where the COMMIT_EDITMSG can be found.

Comments on my implementation are welcome, I'm happy to change it if needed 🙂

Motivation and Context

This change is required in order to support linting the commits of git sub modules.

Fixes #448

Usage examples

n/a

How Has This Been Tested?

Tests are missing from this PR (at least for now) as it requires an update to the @commitlint/test package (support for sub modules in the git helper has to be added). However, I did create a test locally with a rough outline of what the update to the @commitlint/test might look like. You can find the required changes in this gist, simply checkout on this branch and replace the local files with the two in the gist and you can test the new feature.

The reason I opened this PR first is because I wanted to be sure this would be included before updating the @commitlint/test code and adding something that wasn't really needed...

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes. (see "How Has This Been Tested?" for details)
• All new and existing tests passed.