Add tree-sitter-git-diff (#1373)

* add submodule on tree-sitter-git-diff

* add git-diff highlights

* inject git-diff into git-commit

* update tree-sitter-git-commit with fix for bad diff case

* add git-diff to language support docs

* include-children in diff injections

This ensures that children nodes of $.message are included in the
injection, such as $.user or issue/pr numbers. Without this change,
diffs containing '#' or '@' characters can trip up the injection and
be parsed separately.

See https://github.com/helix-editor/helix/pull/1373#issuecomment-1001215629

* set diff language's scope as source.diff

0 files changed, 0 insertions, 0 deletions