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/repository/branch.go

@@ -526,7 +526,7 @@ func UpdateBranch(ctx context.Context, repo *repo_model.Repository, gitRepo *git
 		return nil
 	}
 
-	isForcePush, err := newCommit.IsForcePush(branch.CommitID)
+	isForcePush, err := newCommit.IsForcePush(ctx, gitRepo, branch.CommitID)
 	if err != nil {
 		return err
 	}
@@ -811,7 +811,7 @@ func GetBranchDivergingInfo(ctx reqctx.RequestContext, baseRepo *repo_model.Repo
 		if err != nil {
 			return nil, err
 		}
-		hasPreviousCommit, _ := headCommit.HasPreviousCommit(baseCommitID)
+		hasPreviousCommit, _ := headCommit.HasPreviousCommit(ctx, headGitRepo, baseCommitID)
 		info.BaseHasNewCommits = !hasPreviousCommit
 		return info, nil
 	}