Add lexer for git range-diff "dual-color" output

[phil-blain]

Please note: New lexers are contributed to Rouge from the community. Your request will help the community in developing a lexer but it does not mean that anyone will develop the lexer.

The name of the language

git range-diff: https://git-scm.com/docs/git-range-diff, specifically the "dual-color" output: https://git-scm.com/docs/git-range-diff#_options.

Implementation in other libraries

None that I know of.

Additional context
Since Rouge is used on both GitLab and GitHub as far as I know, this lexer would be quite useful to compare iterations of a branch in the context of a merge request / pull request.