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

services/markup/renderhelper_codepreview.go

@@ -62,7 +62,7 @@ func renderRepoFileCodePreview(ctx context.Context, opts markup.RenderCodePrevie
 	}
 
 	language, _ := languagestats.GetFileLanguage(ctx, gitRepo, opts.CommitID, opts.FilePath)
-	blob, err := commit.GetBlobByPath(opts.FilePath)
+	blob, err := commit.GetBlobByPath(ctx, gitRepo, opts.FilePath)
 	if err != nil {
 		return "", err
 	}