Regression of the ctx removal from "git.Repository" struct (the old code
was already wrong and can still cause 500, the "ctx removal" just makes
the problem easier to reproduce).
Merge duplicate code.
Reviewd by codex: no actionable findings.
1. Storing "ctx" in a long-living object is wrong
2. Make the commit & tree cacheable (for the future performance
optimization)
3. Also fix some bad designs like `// FIXME: bad design, this field can
be nil if the commit is from "last commit cache"`
ref:
* #33893
wxiaoguang