ratchetFrom 'origin/main' is confusing when 'origin/main' moves

[nedtwigg]

The setup:

• you have a repo with two files, mine.txt and untouched.txt
• you have configured them with ratchetFrom 'origin/main'
• you are using licenseHeader '(C) $YEAR'
• all files have (C) 2019, but today's year is now 2020

So far so good:

• you start a feature branch off of main
• you change mine.txt
• you run spotlessApply which changes the copyright header on mine.txt from 2019 to 2020, but leaves untouched.txt alone since you have not changed it
• for your local branch, spotlessCheck is passing

The problem:

• the team merges some PR's to origin/main which change untouched.txt
• you do a git fetch 'origin/main'
• all of a sudden, spotlessCheck will fail for your local branch, even though you didn't make any changes
• if you run spotlessApply, it will update the copyright header on untouched.txt from 2019 to 2020
• now you have a merge conflict, and diff noise in your PR

Workarounds:

• instead of using ratchetFrom 'origin/master', do ratchetFrom 'immutable-tag-like-v1.0.0'
• OR, if Spotless is forcing you to change files you haven't changed, do git merge origin/main, so that you aren't duplicating changes

Solution:

• For mutable branches like origin/master, Spotless should look for the merge-base, not just a naive-diff between origin/master and the WC.

[nedtwigg]

Released in plugin-maven 2.0.0 and plugin-gradle 4.5.0.