feat(pulls): support per-commit review (#39326)

Opening a single commit of a PR let you look, but not review. The Review
button was disabled and there was no way to reach the next or previous
commit. This finishes https://github.com/go-gitea/gitea/pull/37475,
which only reached the head commit.

- Reviews can be submitted from a single commit, a commit range stays
disabled
- A bar above a single-commit diff shows the commit, its position in the
PR and prev/next buttons
- The commit dropdown marks the commits on screen
- Timeline commits open the PR's commit view, commits force-pushed out
of the PR redirect to the repository commit page

Assisted-by: Claude Code (Fable 5.1)
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/pull.go

@@ -6,11 +6,13 @@
 package repo
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"html"
 	"html/template"
 	"net/http"
+	"slices"
 	"strconv"
 	"strings"
 	"time"
@@ -676,6 +678,26 @@ func indexCommit(commits []*git.Commit, commitID string) *git.Commit {
 	return nil
 }
 
+// setSingleCommitNavContext exposes the reviewed commit and its neighbors.
+func setSingleCommitNavContext(ctx *context.Context, commits []*git.Commit, commit *git.Commit) {
+	idx := slices.IndexFunc(commits, func(c *git.Commit) bool { return bytes.Equal(c.ID.RawValue(), commit.ID.RawValue()) })
+	if idx < 0 {
+		return
+	}
+
+	ctx.Data["SingleCommit"] = commit
+	ctx.Data["SingleCommitIndex"] = len(commits) - idx
+	ctx.Data["SingleCommitCount"] = len(commits)
+
+	// The commit list is newest-first while a reviewer walks it oldest-first, so the indexes are mirrored.
+	if idx+1 < len(commits) {
+		ctx.Data["SingleCommitPrevID"] = commits[idx+1].ID.String()
+	}
+	if idx > 0 {
+		ctx.Data["SingleCommitNextID"] = commits[idx-1].ID.String()
+	}
+}
+
 // ViewPullFiles render pull request changed files list page
 func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 	var err error
@@ -703,11 +725,17 @@ func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 	}
 
 	headCommitID := prCompareInfo.HeadCommitID
-	isSingleCommit := beforeCommitID == "" && afterCommitID != ""
-	// FIXME: when afterCommitID==headCommitID, isSingleCommit and isShowAllCommits can be both true, which doesn't seem right
-	isShowAllCommits := (beforeCommitID == "" || beforeCommitID == prCompareInfo.CompareBase) && (afterCommitID == "" || afterCommitID == headCommitID)
+	// show all commits when:
+	// * both before and after commit IDs are empty (default view)
+	// * before commit ID is the PR's base commit and after commit ID is the PR's head commit (default view)
+	// * there is only one commit in the PR and after commit ID is the PR's head commit (single-commit view)
+	isShowAllCommits := (beforeCommitID == "" && afterCommitID == "") ||
+		(beforeCommitID == prCompareInfo.CompareBase && afterCommitID == headCommitID) ||
+		(beforeCommitID == "" && afterCommitID == headCommitID && len(prCompareInfo.Commits) == 1)
+	isSingleCommit := beforeCommitID == "" && afterCommitID != "" && !isShowAllCommits
 
-	ctx.Data["IsShowingOnlySingleCommit"] = isSingleCommit
+	// it's questionable why single commit review can be submitted, but the range commits review cannot be submitted
+	ctx.Data["CanSubmitReview"] = isShowAllCommits || isSingleCommit
 	ctx.Data["IsShowingAllCommits"] = isShowAllCommits
 
 	// "commits list" is half-open, half-closed: (base, head]
@@ -723,6 +751,10 @@ func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 			return
 		}
 	}
+	if afterCommit == nil && isSingleCommit {
+		ctx.Redirect(ctx.Repo.RepoLink + "/commit/" + afterCommitID) // timeline commits outlive force pushes
+		return
+	}
 	if afterCommit == nil {
 		ctx.NotFound(nil)
 		return
@@ -756,6 +788,11 @@ func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 	ctx.Data["CompareInfo"] = prCompareInfo
 	ctx.Data["AfterCommitID"] = afterCommitID
 	ctx.Data["BeforeCommitID"] = beforeCommitID
+	ctx.Data["ReviewCommitID"] = afterCommitID // the review happens on the "after commit" (the latest commit in the range)
+
+	if isSingleCommit {
+		setSingleCommitNavContext(ctx, prCompareInfo.Commits, afterCommit)
+	}
 
 	maxLines, maxFiles := setting.Git.MaxGitDiffLines, setting.Git.MaxGitDiffFiles
 	files := ctx.FormStrings("files")

routers/web/repo/pull_review.go

@@ -56,6 +56,7 @@ func RenderNewCodeCommentForm(ctx *context.Context) {
 		return
 	}
 	ctx.Data["AfterCommitID"] = pullHeadCommitID
+	ctx.Data["ReviewCommitID"] = pullHeadCommitID // it's questionable why it only needs the head commit ID but not correct "AfterCommitID" (single commit review)
 	upload.AddUploadContext(ctx, "comment")
 	ctx.HTML(http.StatusOK, tplNewComment)
 }
@@ -209,6 +210,7 @@ func renderConversation(ctx *context.Context, comment *issues_model.Comment, ori
 		return
 	}
 	ctx.Data["AfterCommitID"] = pullHeadCommitID
+	ctx.Data["ReviewCommitID"] = pullHeadCommitID // it's questionable why it only needs the head commit ID but not correct "AfterCommitID" (single commit review)
 	ctx.Data["CanBlockUser"] = func(blocker, blockee *user_model.User) bool {
 		return user_service.CanBlockUser(ctx, ctx.Doer, blocker, blockee)
 	}