Unfortunately, we can't completely remove the ctx from git.Repository,
because the CatFileBatch still heavily depends on a parent context.
If we remove the Repository ctx, the CatFileBatch will become a mess and
create a lot of unnecessary git processes.
http://localhost:3000/-/admin/monitor/perftrace
* Before: open a repo home, dozens of git processes (duplicate cat-file)
* After: only a few (no duplicate cat-file)
* Implemented calling git diff-tree
* Ensures wrapper function is called with valid arguments
* Parses output into go struct, using strong typing when possible
wxiaoguang