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/indexer/code/indexer.go

@@ -13,6 +13,7 @@ import (
 
 	"gitea.dev/models/db"
 	repo_model "gitea.dev/models/repo"
+	"gitea.dev/modules/gitrepo"
 	"gitea.dev/modules/graceful"
 	"gitea.dev/modules/indexer"
 	"gitea.dev/modules/indexer/code/bleve"
@@ -73,11 +74,17 @@ func index(ctx context.Context, indexer internal.Indexer, repoID int64) error {
 		return nil
 	}
 
+	gitRepo, closer, err := gitrepo.RepositoryFromContextOrOpen(ctx, repo)
+	if err != nil {
+		return err
+	}
+	defer closer.Close()
+
 	sha, err := getDefaultBranchSha(ctx, repo)
 	if err != nil {
 		return err
 	}
-	changes, err := getRepoChanges(ctx, repo, sha)
+	changes, err := getRepoChanges(ctx, repo, gitRepo, sha)
 	if err != nil {
 		return err
 	} else if changes == nil {