Only format/check files which have changed since git ref

[mhagnumdw]

Add optional to only format/check files which have changed since git ref (eg: origin/main). This helps with existing projects that are trying to apply some code standardization.

[ppalaga]

I'd be open to review a PR, that would

• Introduce an SPI for additional excludes
• Introduce an implementation of that SPI in a separate maven module that could optionally be added as a dependency of the plugin

I think it is better if the vanilla plugin stays independent of jgit.