poor performance when injecting a large diff

[the-mikedavis]

Committing a verbose commit (shows diff after scissors) can slow down an editor using tree-sitter-git-commit quite a bit if the diff is very large.

This is probably because:

• this grammar has pretty small message nodes which become numerous on a big commit file
• tree-sitter-git-diff is currently injected within those message nodes one-by-one

[the-mikedavis]

Even a 8000 line verbose commit message is very fast after d046669. I expect that the slow-down was that incremental updates to the injected layers became very expensive when there were thousands of injected git-diff lines.