Add merge style fast-forward-only (#28954)

With this option, it is possible to require a linear commit history with
the following benefits over the next best option `Rebase+fast-forward`:
The original commits continue existing, with the original signatures
continuing to stay valid instead of being rewritten, there is no merge
commit, and reverting commits becomes easier.

Closes #24906

services/forms/repo_form.go

@@ -151,6 +151,7 @@ type RepoSettingForm struct {
 	PullsAllowRebase                      bool
 	PullsAllowRebaseMerge                 bool
 	PullsAllowSquash                      bool
+	PullsAllowFastForwardOnly             bool
 	PullsAllowManualMerge                 bool
 	PullsDefaultMergeStyle                string
 	EnableAutodetectManualMerge           bool
@@ -598,8 +599,8 @@ func (f *InitializeLabelsForm) Validate(req *http.Request, errs binding.Errors)
 // swagger:model MergePullRequestOption
 type MergePullRequestForm struct {
 	// required: true
-	// enum: merge,rebase,rebase-merge,squash,manually-merged
-	Do                     string `binding:"Required;In(merge,rebase,rebase-merge,squash,manually-merged)"`
+	// enum: merge,rebase,rebase-merge,squash,fast-forward-only,manually-merged
+	Do                     string `binding:"Required;In(merge,rebase,rebase-merge,squash,fast-forward-only,manually-merged)"`
 	MergeTitleField        string
 	MergeMessageField      string
 	MergeCommitID          string // only used for manually-merged