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

templates/repo/issue/view_content/pull_merge_instruction.tmpl

@@ -35,6 +35,10 @@
 			<div>git checkout {{.PullRequest.BaseBranch}}</div>
 			<div>git merge --squash {{$localBranch}}</div>
 		</div>
+		<div class="gt-hidden" data-pull-merge-style="fast-forward-only">
+			<div>git checkout {{.PullRequest.BaseBranch}}</div>
+			<div>git merge --ff-only {{$localBranch}}</div>
+		</div>
 		<div class="gt-hidden" data-pull-merge-style="manually-merged">
 			<div>git checkout {{.PullRequest.BaseBranch}}</div>
 			<div>git merge {{$localBranch}}</div>