Document or fix linting on shallow clones

[marionebl]

Shallow cloned repository yield errors via git-raw-commits. conventional-changelog-lint should either document how to circumvent these limitations or support commit listing on shallow clones, too.

see #6

[kumar303]

Here's a workaround for TravisCI (which does shallow clones): #6 (comment)

[kumar303]

My workaround had an issue with tag commits since those result in a detached head. My current workaround is to do this in my before_install TravisCI yaml section:

git remote set-branches origin master && git fetch
git checkout master
# Go back to the original commit that TravisCI checked out:
git checkout -

I'm not sure exactly why but this makes the master branch visible to the linter.

[marionebl]

After chewing around on this for quite a while I'll go with the following approach:

0. There is no way to get the git log messages from a shallow clone
1. Document the need to pull in the full history for shallow clones. Provide TravisCI snippet.
2. Detect shallow clones when running getMessages and print a warning throw with a link to said documentation