Add checkbox to delete pull branch after successful merge (#16049)

* Add checkbox to delete pull branch after successful merge

* Omit DeleteBranchAfterMerge field in json

* Log a warning instead of error when PR head branch deleted

* Add DefaultDeleteBranchAfterMerge to PullRequestConfig

* Add support for delete_branch_after_merge via API

* Fix for API: the branch should be deleted from the HEAD repo

If head and base repo are the same, reuse the already opened ctx.Repo.GitRepo

* Don't delegate to CleanupBranch, only reuse branch deletion code

CleanupBranch contains too much logic that has already been performed by the Merge

* Reuse gitrepo in MergePullRequest

Co-authored-by: Andrew Thornton <art27@cantab.net>

models/repo_unit.go

@@ -91,14 +91,15 @@ func (cfg *IssuesConfig) ToDB() ([]byte, error) {
 
 // PullRequestsConfig describes pull requests config
 type PullRequestsConfig struct {
-	IgnoreWhitespaceConflicts bool
-	AllowMerge                bool
-	AllowRebase               bool
-	AllowRebaseMerge          bool
-	AllowSquash               bool
-	AllowManualMerge          bool
-	AutodetectManualMerge     bool
-	DefaultMergeStyle         MergeStyle
+	IgnoreWhitespaceConflicts     bool
+	AllowMerge                    bool
+	AllowRebase                   bool
+	AllowRebaseMerge              bool
+	AllowSquash                   bool
+	AllowManualMerge              bool
+	AutodetectManualMerge         bool
+	DefaultDeleteBranchAfterMerge bool
+	DefaultMergeStyle             MergeStyle
 }
 
 // FromDB fills up a PullRequestsConfig from serialized format.