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/render.go

@@ -19,7 +19,7 @@ func RenderFile(ctx *context.Context) {
 	var blob *git.Blob
 	var err error
 	if ctx.Repo.TreePath != "" {
-		blob, err = ctx.Repo.Commit.GetBlobByPath(ctx.Repo.TreePath)
+		blob, err = ctx.Repo.Commit.GetBlobByPath(ctx, ctx.Repo.GitRepo, ctx.Repo.TreePath)
 	} else {
 		blob, err = ctx.Repo.GitRepo.GetBlob(ctx.PathParam("sha"))
 	}