fix(pulls): add branch-name option for DEFAULT_TITLE_SOURCE (#38356)

Adds a new `branch-name` value for the `[repository.pull-request]`
`DEFAULT_TITLE_SOURCE` setting that always uses the normalized branch
name as the PR title, regardless of commit count.

Fix #38317

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/repository.go

@@ -22,6 +22,7 @@ const (
 const (
 	RepoPRTitleSourceFirstCommit = "first-commit"
 	RepoPRTitleSourceAuto        = "auto"
+	RepoPRTitleSourceBranchName  = "branch-name"
 )
 
 // ItemsPerPage maximum items per page in forks, watchers and stars of a repo