Allow deleting a merge commit

For non-merge commits we change "pick" to "drop" when we delete them. We do this
so that we can use the same code for dropping a commit no matter whether we are
in an interactive rebase or not. (If we aren't, we could just as well delete the
pick line from the todo list instead of setting it to "drop", but if we are, it
is better to keep the line around so that the user can change it back to "pick"
if they change their mind.)

However, merge commits can't be changed to "drop", so we have to delete them
from the todo file. We add a new daemon instruction that does this.

We still don't allow deleting a merge commit from within an interactive rebase.
The reason is that we don't show the "label" and "reset" todos in lazygit, so
deleting a merge commit would leave the commits from the branch that is being
merged in the list as "pick" commits, with no indication that they are going to
be dropped because they are on a different branch, and the merge commit that
would have brought them in is gone. This could be very confusing.

pkg/utils/rebase_todo.go

@@ -290,3 +290,29 @@ func RemoveUpdateRefsForCopiedBranch(fileName string, commentChar byte) error {
 func isRenderedTodo(t todo.Todo) bool {
 	return t.Commit != "" || t.Command == todo.UpdateRef
 }
+
+func DropMergeCommit(fileName string, hash string, commentChar byte) error {
+	todos, err := ReadRebaseTodoFile(fileName, commentChar)
+	if err != nil {
+		return err
+	}
+
+	newTodos, err := dropMergeCommit(todos, hash)
+	if err != nil {
+		return err
+	}
+
+	return WriteRebaseTodoFile(fileName, newTodos, commentChar)
+}
+
+func dropMergeCommit(todos []todo.Todo, hash string) ([]todo.Todo, error) {
+	isMerge := func(t todo.Todo) bool {
+		return t.Command == todo.Merge && t.Flag == "-C" && equalHash(t.Commit, hash)
+	}
+	if lo.CountBy(todos, isMerge) != 1 {
+		return nil, fmt.Errorf("Expected exactly one merge commit with hash %s", hash)
+	}
+
+	_, idx, _ := lo.FindIndexOf(todos, isMerge)
+	return slices.Delete(todos, idx, idx+1), nil
+}