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/feed/branch.go

@@ -21,7 +21,7 @@ func ShowBranchFeed(ctx *context.Context, repo *repo.Repository, formatType stri
 	var commits []*git.Commit
 	var err error
 	if ctx.Repo.Commit != nil {
-		commits, err = ctx.Repo.Commit.CommitsByRange(0, 10, "", "", "")
+		commits, err = ctx.Repo.Commit.CommitsByRange(ctx.Repo.GitRepo, 0, 10, "", "", "")
 		if err != nil {
 			ctx.ServerError("ShowBranchFeed", err)
 			return