Filter out merge commits when generating todo changes in InteractiveRebase

We will need this because under some conditions we are going to use this
function to edit a range of commits, and we can't set merge commits to "edit".
This corresponds to the code in startInteractiveRebaseWithEdit which has similar
logic.

It is a bit unfortunate that we will have these two different ways of setting
todos to edit: startInteractiveRebaseWithEdit does it after stopping in the
rebase, in the Then function of its refresh, but InteractiveRebase does it in
the daemon with a ChangeTodoActionsInstruction. It still makes sense though,
given how InteractiveRebase works.

This not only affects "edit", but also "drop", "fixup", and "squash".
Previously, when trying to use these for a range selection that includes a merge
commit, they would fail with the cryptic error message "Some todos not found in
git-rebase-todo"; now they simply exclude the merge commit. I'm not sure if one
is better or worse than the other, and we should probably simply disable the
commands when a merge commit is selected, but that's out of scope in this PR.

Author: stefanhaller

pkg/commands/git_commands/rebase.go

@@ -145,11 +145,11 @@ func (self *RebaseCommands) InteractiveRebase(commits []*models.Commit, startIdx
 
 	baseHashOrRoot := getBaseHashOrRoot(commits, baseIndex)
 
-	changes := lo.Map(commits[startIdx:endIdx+1], func(commit *models.Commit, _ int) daemon.ChangeTodoAction {
+	changes := lo.FilterMap(commits[startIdx:endIdx+1], func(commit *models.Commit, _ int) (daemon.ChangeTodoAction, bool) {
 		return daemon.ChangeTodoAction{
 			Hash:      commit.Hash,
 			NewAction: action,
-		}
+		}, !commit.IsMerge()
 	})
 
 	self.os.LogCommand(logTodoChanges(changes), false)