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

models/repo/git.go

@@ -21,6 +21,8 @@ const (
 	MergeStyleRebaseMerge MergeStyle = "rebase-merge"
 	// MergeStyleSquash squash commits into single commit before merging
 	MergeStyleSquash MergeStyle = "squash"
+	// MergeStyleFastForwardOnly fast-forward merge if possible, otherwise fail
+	MergeStyleFastForwardOnly MergeStyle = "fast-forward-only"
 	// MergeStyleManuallyMerged pr has been merged manually, just mark it as merged directly
 	MergeStyleManuallyMerged MergeStyle = "manually-merged"
 	// MergeStyleRebaseUpdate not a merge style, used to update pull head by rebase

models/repo/repo_unit.go

@@ -122,6 +122,7 @@ type PullRequestsConfig struct {
 	AllowRebase                   bool
 	AllowRebaseMerge              bool
 	AllowSquash                   bool
+	AllowFastForwardOnly          bool
 	AllowManualMerge              bool
 	AutodetectManualMerge         bool
 	AllowRebaseUpdate             bool
@@ -148,6 +149,7 @@ func (cfg *PullRequestsConfig) IsMergeStyleAllowed(mergeStyle MergeStyle) bool {
 		mergeStyle == MergeStyleRebase && cfg.AllowRebase ||
 		mergeStyle == MergeStyleRebaseMerge && cfg.AllowRebaseMerge ||
 		mergeStyle == MergeStyleSquash && cfg.AllowSquash ||
+		mergeStyle == MergeStyleFastForwardOnly && cfg.AllowFastForwardOnly ||
 		mergeStyle == MergeStyleManuallyMerged && cfg.AllowManualMerge
 }