Add no-ff merge option

This will put whatever git's default merge variant is as the first menu item,
and add a second item which is the opposite (no-ff if the default is ff, and
vice versa).

If users prefer to always have the same option first no matter whether it's
applicable, they can make ff always appear first by setting git's "merge.ff"
config to "true" or "only", or by setting lazygit's "git.merging.args" config to
"--ff" or "--ff-only"; if they want no-ff to appear first, they can do that by
setting git's "merge.ff" config to "false", or by setting lazygit's
"git.merging.args" config to "--no-ff". Which of these they choose depends on
whether they want the config to also apply to other git clients including the
cli, or only to lazygit.

pkg/commands/git_commands/branch.go

@@ -285,6 +285,16 @@ func (self *BranchCommands) Merge(branchName string, variant MergeVariant) error
 	return self.cmd.New(cmdArgs).Run()
 }
 
+// Returns whether refName can be fast-forward merged into the current branch
+func (self *BranchCommands) CanDoFastForwardMerge(refName string) bool {
+	cmdArgs := NewGitCmd("merge-base").
+		Arg("--is-ancestor").
+		Arg("HEAD", refName).
+		ToArgv()
+	err := self.cmd.New(cmdArgs).DontLog().Run()
+	return err == nil
+}
+
 // Only choose between non-empty, non-identical commands
 func (self *BranchCommands) allBranchesLogCandidates() []string {
 	return lo.Uniq(lo.WithoutEmpty(self.UserConfig().Git.AllBranchesLogCmds))

pkg/commands/git_commands/config.go

@@ -97,6 +97,10 @@ func (self *ConfigCommands) GetRebaseUpdateRefs() bool {
 	return self.gitConfig.GetBool("rebase.updateRefs")
 }
 
+func (self *ConfigCommands) GetMergeFF() string {
+	return self.gitConfig.Get("merge.ff")
+}
+
 func (self *ConfigCommands) DropConfigCache() {
 	self.gitConfig.DropCache()
 }