Git diff: add config option relative

After the tag.forceSignAnnotated configuration option, I've worked on git to add a new configuration option already present on the diff command line --relative.

Sometimes I'm working on repositories with multiple projects and I wanted to have the diff only on the current project per default. So naively I set the diff.relative to true on the gitconfig file and it did not work, then checked to documentation (usually I check this first, but I'm a bit wacky sometimes).

Next step was to submit a patch and thanks to the interest and reviews of maintainers/contributors it is now merged.