Add DEFAULT_TITLE_SOURCE setting for pull request title default behavior (#37465)

Adds a new `DEFAULT_TITLE_SOURCE` option under
`[repository.pull-request]` with three values:

- `first-commit` (default): uses the oldest commit summary, current
behavior since v1.26
- `auto`: normalizes branch name as title for multi-commit PRs (just
like GitHub), use commit summary for single-commit PRs

Closes: #37463
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Nicolas <bircni@icloud.com>

modules/setting/repository.go

@@ -18,6 +18,12 @@ const (
 	RepoCreatingPublic             = "public"
 )
 
+// enumerates the values for [repository.pull-request] DEFAULT_TITLE_SOURCE
+const (
+	RepoPRTitleSourceFirstCommit = "first-commit"
+	RepoPRTitleSourceAuto        = "auto"
+)
+
 // ItemsPerPage maximum items per page in forks, watchers and stars of a repo
 const ItemsPerPage = 40
 
@@ -89,6 +95,7 @@ var (
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
 			DefaultDeleteBranchAfterMerge            bool
+			DefaultTitleSource                       string
 		} `ini:"repository.pull-request"`
 
 		// Issue Setting
@@ -213,6 +220,7 @@ var (
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
 			DefaultDeleteBranchAfterMerge            bool
+			DefaultTitleSource                       string
 		}{
 			WorkInProgressPrefixes: []string{"WIP:", "[WIP]"},
 			// Same as GitHub. See
@@ -229,6 +237,7 @@ var (
 			AddCoCommitterTrailers:                   true,
 			RetargetChildrenOnMerge:                  true,
 			DelayCheckForInactiveDays:                7,
+			DefaultTitleSource:                       RepoPRTitleSourceFirstCommit,
 		},
 
 		// Issue settings