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

modules/git/error.go

@@ -96,7 +96,7 @@ func (err ErrBranchNotExist) Unwrap() error {
 	return util.ErrNotExist
 }
 
-// ErrPushOutOfDate represents an error if merging fails due to unrelated histories
+// ErrPushOutOfDate represents an error if merging fails due to the base branch being updated
 type ErrPushOutOfDate struct {
 	StdOut string
 	StdErr string