Support for new requirements.txt output from pip-compile

[cancan101]

As of pip-tools vs 5.5.0, the format of the requirements.txt file generated by pip-compile has changed slightly in that each comment is on its own line. See for example: jazzband/pip-tools#1297 (comment).

This means that when dependabot runs, it generates a huge diff to the requirements.txt file if the checked in file was generated from a newer version of pip-compile.

[jurre]

I think #2943 should fix this @cancan101

[jerr0328]

Now the PR is #2985