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/shared/user/header.go

@@ -122,7 +122,7 @@ func FindOwnerProfileReadme(ctx *context.Context, doer *user_model.User, optProf
 		return nil, nil
 	}
 
-	profileReadmeBlob, _ = commit.GetBlobByPath("README.md") // no need to handle this error
+	profileReadmeBlob, _ = commit.GetBlobByPath(ctx, profileGitRepo, "README.md") // no need to handle this error
 	return profileDbRepo, profileReadmeBlob
 }