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>

templates/swagger/v1_json.tmpl

@@ -14058,6 +14058,11 @@
           "type": "string",
           "x-go-name": "DefaultBranch"
         },
+        "default_delete_branch_after_merge": {
+          "description": "set to `true` to delete pr branch after merge by default",
+          "type": "boolean",
+          "x-go-name": "DefaultDeleteBranchAfterMerge"
+        },
         "default_merge_style": {
           "description": "set to a merge style to be used by this repository: \"merge\", \"rebase\", \"rebase-merge\", or \"squash\". `has_pull_requests` must be `true`.",
           "type": "string",
@@ -15137,6 +15142,10 @@
         "MergeTitleField": {
           "type": "string"
         },
+        "delete_branch_after_merge": {
+          "type": "boolean",
+          "x-go-name": "DeleteBranchAfterMerge"
+        },
         "force_merge": {
           "type": "boolean",
           "x-go-name": "ForceMerge"