refactor: decouple git.Repository(ctx) from git.Commit & git.Tree (#38464)

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

routers/web/repo/middlewares.go

@@ -22,7 +22,7 @@ func SetEditorconfigIfExists(ctx *context.Context) {
 		return
 	}
 
-	ec, _, err := ctx.Repo.GetEditorconfig()
+	ec, _, err := ctx.Repo.GetEditorconfig(ctx)
 	if err != nil {
 		// it used to check `!git.IsErrNotExist(err)` and create a system notice, but it is quite annoying and useless
 		// because network errors also happen frequently, so we just ignore it