Add the ability to populate header years from git history.

[nedtwigg]

If you've setup a license header with $YEAR, you can do gradlew spotlessApply -P spotlessSetLicenseHeaderYearsFromGitHistory=true, and it will change every license header to match the output of:

• oldest year: git log --follow --diff-filter=A --find-renames=40%
• newest year: git log --max-count=1

[nedtwigg]

It generates too much diff noise for a project with ongoing PRs such as this one, but as an example, this is the effect of running the command on this repository: dd279c3

You could adopt this slowly over time with ratchetFrom 'origin/master', but it is so slow and it disables up-to-date checking, so it's really only good for "format-the-world" cases. It took 1m 6s to run on this repository on my 2018 Macbook Pro (quad core 16GB RAM). Running on a larger project (~3k files with ~10 years of history) took 10m on a quad-core i7-7700HQ (VR-spec rig from 2017). But very useful for quickly getting an internal project ready to opensource with accurate header info.