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/api/v1/repo/tree.go

@@ -62,7 +62,7 @@ func GetTree(ctx *context.APIContext) {
 		return
 	}
 	if tree, err := files_service.GetTreeBySHA(ctx, ctx.Repo.Repository, ctx.Repo.GitRepo, sha, ctx.FormInt("page"), ctx.FormInt("per_page"), ctx.FormBool("recursive")); err != nil {
-		ctx.APIError(http.StatusBadRequest, err.Error())
+		ctx.APIErrorAuto(err)
 	} else {
 		ctx.SetTotalCountHeader(int64(tree.TotalCount))
 		ctx.JSON(http.StatusOK, tree)