Java CLI to format parts of files based on git diff

[iamdanfox]

Before this PR

One of the main goals of this whole endeavour is to make excavator code-changes pass checkstyle. To achieve this, we don't actually need to reformat people's entire projects, just ensure that the PRs we make have nice linebreaks, don't go past 120 chars etc.

(Full adoption of the formatter everywhere comes with its own benefits, but it's almost orthogonal).

After this PR

==COMMIT_MSG==
The FormatDiffCli class reads the output of git diff -U0 and runs the formatter on modified lines only.
==COMMIT_MSG==

The plan is to run this from a gradle task in baseline, and conceivably even as a pre-commit hook!! (Here's the original python one)

Possible downsides?

• the original python script that does this had a bunch more configurability - maybe we'll end up wanting this? not sure
• should probably try and be forwards compatible?

[changelog-app]

Generate changelog in changelog/@unreleased

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

A new FormatDiffCli class reads the output of git diff -U0 and runs the formatter on modified lines only.

Check the box to generate changelog(s)

• Generate changelog entry