Move database operations of merging a pull request to post receive hook and add a transaction (#30805)

Merging PR may fail because of various problems. The pull request may
have a dirty state because there is no transaction when merging a pull
request. ref
https://github.com/go-gitea/gitea/pull/25741#issuecomment-2074126393

This PR moves all database update operations to post-receive handler for
merging a pull request and having a database transaction. That means if
database operations fail, then the git merging will fail, the git client
will get a fail result.

There are already many tests for pull request merging, so we don't need
to add a new one.

---------

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

services/contexttest/context_tests.go

@@ -94,6 +94,19 @@ func MockAPIContext(t *testing.T, reqPath string) (*context.APIContext, *httptes
 	return ctx, resp
 }
 
+func MockPrivateContext(t *testing.T, reqPath string) (*context.PrivateContext, *httptest.ResponseRecorder) {
+	resp := httptest.NewRecorder()
+	req := mockRequest(t, reqPath)
+	base, baseCleanUp := context.NewBaseContext(resp, req)
+	base.Data = middleware.GetContextData(req.Context())
+	base.Locale = &translation.MockLocale{}
+	ctx := &context.PrivateContext{Base: base}
+	_ = baseCleanUp // during test, it doesn't need to do clean up. TODO: this can be improved later
+	chiCtx := chi.NewRouteContext()
+	ctx.Base.AppendContextValue(chi.RouteCtxKey, chiCtx)
+	return ctx, resp
+}
+
 // LoadRepo load a repo into a test context.
 func LoadRepo(t *testing.T, ctx gocontext.Context, repoID int64) {
 	var doer *user_model.User