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/private/hook_verification.go

@@ -64,7 +64,7 @@ func readAndVerifyCommit(sha string, repo *git.Repository, env []string) error {
 	return cmd.WithEnv(env).
 		WithDir(repo.Path).
 		WithPipelineFunc(func(ctx gitcmd.Context) error {
-			commit, err := git.CommitFromReader(repo, commitID, stdoutReader)
+			commit, err := git.CommitFromReader(commitID, stdoutReader)
 			if err != nil {
 				return err
 			}