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

modules/git/languagestats/language_stats_gogit.go

@@ -7,6 +7,7 @@ package languagestats
 
 import (
 	"bytes"
+	"context"
 	"io"
 
 	"gitea.dev/modules/analyze"
@@ -21,7 +22,7 @@ import (
 )
 
 // GetLanguageStats calculates language stats for git repository at specified commit
-func GetLanguageStats(repo *git_module.Repository, commitID string) (map[string]int64, error) {
+func GetLanguageStats(_ context.Context, repo *git_module.Repository, commitID string) (map[string]int64, error) {
 	r, err := git.PlainOpen(repo.Path)
 	if err != nil {
 		return nil, err