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/api/v1/repo/repo.go

@@ -885,6 +885,7 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 					AllowRebase:                   true,
 					AllowRebaseMerge:              true,
 					AllowSquash:                   true,
+					AllowFastForwardOnly:          true,
 					AllowManualMerge:              true,
 					AutodetectManualMerge:         false,
 					AllowRebaseUpdate:             true,
@@ -911,6 +912,9 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 			if opts.AllowSquash != nil {
 				config.AllowSquash = *opts.AllowSquash
 			}
+			if opts.AllowFastForwardOnly != nil {
+				config.AllowFastForwardOnly = *opts.AllowFastForwardOnly
+			}
 			if opts.AllowManualMerge != nil {
 				config.AllowManualMerge = *opts.AllowManualMerge
 			}