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

routers/web/repo/issue.go

@@ -1862,6 +1862,8 @@ func ViewIssue(ctx *context.Context) {
 				mergeStyle = repo_model.MergeStyleRebaseMerge
 			} else if prConfig.AllowSquash {
 				mergeStyle = repo_model.MergeStyleSquash
+			} else if prConfig.AllowFastForwardOnly {
+				mergeStyle = repo_model.MergeStyleFastForwardOnly
 			} else if prConfig.AllowManualMerge {
 				mergeStyle = repo_model.MergeStyleManuallyMerged
 			}

routers/web/repo/setting/setting.go

@@ -576,6 +576,7 @@ func SettingsPost(ctx *context.Context) {
 					AllowRebase:                   form.PullsAllowRebase,
 					AllowRebaseMerge:              form.PullsAllowRebaseMerge,
 					AllowSquash:                   form.PullsAllowSquash,
+					AllowFastForwardOnly:          form.PullsAllowFastForwardOnly,
 					AllowManualMerge:              form.PullsAllowManualMerge,
 					AutodetectManualMerge:         form.EnableAutodetectManualMerge,
 					AllowRebaseUpdate:             form.PullsAllowRebaseUpdate,