Change PR commits and diffs to use base repo rather than forked

Change the repository referenced when displaying commits or diffs in
pull request to the base repository.  The forked repository may not be
readable by users who can read the base repository.  See discussion for
(go-gitea#3356).

routers/repo/pull.go

@@ -362,30 +362,23 @@ func ViewPullCommits(ctx *context.Context) {
 	pull := issue.PullRequest
 
 	var commits *list.List
+	var prInfo *git.PullRequestInfo
 	if pull.HasMerged {
-		prInfo := PrepareMergedViewPullInfo(ctx, issue)
-		if ctx.Written() {
-			return
-		} else if prInfo == nil {
-			ctx.NotFound("ViewPullCommits", nil)
-			return
-		}
-		ctx.Data["Username"] = ctx.Repo.Owner.Name
-		ctx.Data["Reponame"] = ctx.Repo.Repository.Name
-		commits = prInfo.Commits
+		prInfo = PrepareMergedViewPullInfo(ctx, issue)
 	} else {
-		prInfo := PrepareViewPullInfo(ctx, issue)
-		if ctx.Written() {
-			return
-		} else if prInfo == nil {
-			ctx.NotFound("ViewPullCommits", nil)
-			return
-		}
-		ctx.Data["Username"] = pull.HeadUserName
-		ctx.Data["Reponame"] = pull.HeadRepo.Name
-		commits = prInfo.Commits
+		prInfo = PrepareViewPullInfo(ctx, issue)
 	}
 
+	if ctx.Written() {
+		return
+	} else if prInfo == nil {
+		ctx.NotFound("ViewPullCommits", nil)
+		return
+	}
+
+	ctx.Data["Username"] = ctx.Repo.Owner.Name
+	ctx.Data["Reponame"] = ctx.Repo.Repository.Name
+	commits = prInfo.Commits
 	commits = models.ValidateCommitsWithEmails(commits)
 	commits = models.ParseCommitsWithSignature(commits)
 	commits = models.ParseCommitsWithStatus(commits, ctx.Repo.Repository)
@@ -420,62 +413,35 @@ func ViewPullFiles(ctx *context.Context) {
 	)
 
 	var headTarget string
+	var prInfo *git.PullRequestInfo
 	if pull.HasMerged {
-		prInfo := PrepareMergedViewPullInfo(ctx, issue)
-		if ctx.Written() {
-			return
-		} else if prInfo == nil {
-			ctx.NotFound("ViewPullFiles", nil)
-			return
-		}
-
-		diffRepoPath = ctx.Repo.GitRepo.Path
-		gitRepo = ctx.Repo.GitRepo
-
-		headCommitID, err := gitRepo.GetRefCommitID(pull.GetGitRefName())
-		if err != nil {
-			ctx.ServerError("GetRefCommitID", err)
-			return
-		}
-
-		startCommitID = prInfo.MergeBase
-		endCommitID = headCommitID
-
-		headTarget = path.Join(ctx.Repo.Owner.Name, ctx.Repo.Repository.Name)
-		ctx.Data["Username"] = ctx.Repo.Owner.Name
-		ctx.Data["Reponame"] = ctx.Repo.Repository.Name
+		prInfo = PrepareMergedViewPullInfo(ctx, issue)
 	} else {
-		prInfo := PrepareViewPullInfo(ctx, issue)
-		if ctx.Written() {
-			return
-		} else if prInfo == nil {
-			ctx.NotFound("ViewPullFiles", nil)
-			return
-		}
+		prInfo = PrepareViewPullInfo(ctx, issue)
+	}
 
-		headRepoPath := models.RepoPath(pull.HeadUserName, pull.HeadRepo.Name)
+	if ctx.Written() {
+		return
+	} else if prInfo == nil {
+		ctx.NotFound("ViewPullFiles", nil)
+		return
+	}
 
-		headGitRepo, err := git.OpenRepository(headRepoPath)
-		if err != nil {
-			ctx.ServerError("OpenRepository", err)
-			return
-		}
+	diffRepoPath = ctx.Repo.GitRepo.Path
+	gitRepo = ctx.Repo.GitRepo
 
-		headCommitID, err := headGitRepo.GetBranchCommitID(pull.HeadBranch)
-		if err != nil {
-			ctx.ServerError("GetBranchCommitID", err)
-			return
-		}
+	headCommitID, err := gitRepo.GetRefCommitID(pull.GetGitRefName())
+	if err != nil {
+		ctx.ServerError("GetRefCommitID", err)
+		return
+	}
 
-		diffRepoPath = headRepoPath
-		startCommitID = prInfo.MergeBase
-		endCommitID = headCommitID
-		gitRepo = headGitRepo
+	startCommitID = prInfo.MergeBase
+	endCommitID = headCommitID
 
-		headTarget = path.Join(pull.HeadUserName, pull.HeadRepo.Name)
-		ctx.Data["Username"] = pull.HeadUserName
-		ctx.Data["Reponame"] = pull.HeadRepo.Name
-	}
+	headTarget = path.Join(ctx.Repo.Owner.Name, ctx.Repo.Repository.Name)
+	ctx.Data["Username"] = ctx.Repo.Owner.Name
+	ctx.Data["Reponame"] = ctx.Repo.Repository.Name
 
 	diff, err := models.GetDiffRangeWithWhitespaceBehavior(diffRepoPath,
 		startCommitID, endCommitID, setting.Git.MaxGitDiffLines,