fix!: add DEFAULT_TITLE_SOURCE setting for pull request title default behavior (#37465) (#37766)

Backport #37465

Make DEFAULT_TITLE_SOURCE default to "auto" like GitHub

---------

Co-authored-by: 0xGREG <28388707+0xGREG@users.noreply.github.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Nicolas <bircni@icloud.com>

routers/web/repo/compare.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"unicode"
 
 	"code.gitea.io/gitea/models/db"
 	git_model "code.gitea.io/gitea/models/git"
@@ -429,13 +430,46 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 	return compareInfo
 }
 
-func prepareNewPullRequestTitleContent(ci *git_service.CompareInfo, commits []*git_model.SignCommitWithStatuses) (title, content string) {
-	title = ci.HeadRef.ShortName()
+// autoTitleFromBranchName humanizes a branch name into a PR title.
+func autoTitleFromBranchName(name string) string {
+	var buf strings.Builder
+	var prevIsSpace bool
+	runes := []rune(name)
+	for i, r := range runes {
+		isSpace := unicode.IsSpace(r)
+		if r == '-' || r == '_' || isSpace {
+			if !prevIsSpace {
+				buf.WriteRune(' ')
+			}
+			prevIsSpace = true
+			continue
+		}
+		if !prevIsSpace && unicode.IsUpper(r) {
+			needSpace := i > 0 && unicode.IsLower(runes[i-1]) || i < len(runes)-1 && unicode.IsLower(runes[i+1])
+			if needSpace {
+				buf.WriteRune(' ')
+			}
+		}
+		buf.WriteRune(unicode.ToLower(r))
+		prevIsSpace = isSpace
+	}
+	out := strings.TrimSpace(buf.String())
+	if out == "" {
+		return out
+	}
+	outRunes := []rune(out)
+	outRunes[0] = unicode.ToUpper(outRunes[0])
+	return string(outRunes)
+}
 
-	if len(commits) > 0 {
+func prepareNewPullRequestTitleContent(ci *git_service.CompareInfo, commits []*git_model.SignCommitWithStatuses, defaultTitleSource string) (title, content string) {
+	useFirstCommitAsTitle := len(commits) == 1 || (defaultTitleSource == setting.RepoPRTitleSourceFirstCommit && len(commits) > 0)
+	if useFirstCommitAsTitle {
 		// the "commits" are from "ShowPrettyFormatLogToList", which is ordered from newest to oldest, here take the oldest one
 		c := commits[len(commits)-1]
 		title = strings.TrimSpace(c.UserCommit.Summary())
+	} else {
+		title = autoTitleFromBranchName(ci.HeadRef.ShortName())
 	}
 
 	if len(commits) == 1 {
@@ -571,7 +605,7 @@ func PrepareCompareDiff(
 	ctx.Data["Commits"] = commits
 	ctx.Data["CommitCount"] = len(commits)
 
-	ctx.Data["title"], ctx.Data["content"] = prepareNewPullRequestTitleContent(ci, commits)
+	ctx.Data["title"], ctx.Data["content"] = prepareNewPullRequestTitleContent(ci, commits, setting.Repository.PullRequest.DefaultTitleSource)
 	ctx.Data["Username"] = ci.HeadRepo.OwnerName
 	ctx.Data["Reponame"] = ci.HeadRepo.Name