Fix amend to operation not working with non-HEAD merge commit (#3510)

- **PR Description**

Resolves #3421

The error is "Expected exactly one original SHA, found 0" but the merge
commit hash (`d6a7a04c626e40071133de26ebe8fdd225caa5c0`) is present in
the rebase TODO file.


![image](https://github.com/jesseduffield/lazygit/assets/13722457/2e6d5fdb-af9f-4eae-9972-8e51a77ba614)

![image](https://github.com/jesseduffield/lazygit/assets/13722457/65dd4b1b-b080-47b0-9079-71c5e0d76cd2)

However, the commit is missed during search because the filter is only
looking for pick commits:
https://github.com/jesseduffield/lazygit/blob/580818e935e19a67f7fe1bbb148224a95781879c/pkg/utils/rebase_todo.go#L238

Checking for merge commits as well fixes the issue.

I believe only pick and merge should be valid here. If already in an
interactive rebase, lazygit only allows amending to the current HEAD
commit. When that happens, this whole interactive rebase logic is
bypassed and lazygit just performs `git commit --amend`:
https://github.com/jesseduffield/lazygit/blob/580818e935e19a67f7fe1bbb148224a95781879c/pkg/gui/controllers/local_commits_controller.go#L668

This is the reason why amending to a HEAD merge commit currently works
whereas non-HEAD does not.

pkg/utils/rebase_todo.go

@@ -235,7 +235,7 @@ func MoveFixupCommitDown(fileName string, originalHash string, fixupHash string,
 
 func moveFixupCommitDown(todos []todo.Todo, originalHash string, fixupHash string) ([]todo.Todo, error) {
  isOriginal := func(t todo.Todo) bool {
- return t.Command == todo.Pick && equalHash(t.Commit, originalHash)
+ return (t.Command == todo.Pick || t.Command == todo.Merge) && equalHash(t.Commit, originalHash)
  }
 
  isFixup := func(t todo.Todo) bool {

pkg/utils/rebase_todo_test.go

@@ -284,7 +284,6 @@ func TestRebaseCommands_moveFixupCommitDown(t *testing.T) {
  expectedErr: nil,
  },
  {
- // TODO: is this something we actually want to support?
  name: "fixup commit is separated from original commit",
  todos: []todo.Todo{
  {Command: todo.Pick, Commit: "original"},
@@ -300,6 +299,22 @@ func TestRebaseCommands_moveFixupCommitDown(t *testing.T) {
  },
  expectedErr: nil,
  },
+ {
+ name: "fixup commit is separated from original merge commit",
+ todos: []todo.Todo{
+ {Command: todo.Merge, Commit: "original"},
+ {Command: todo.Pick, Commit: "other"},
+ {Command: todo.Pick, Commit: "fixup"},
+ },
+ originalHash: "original",
+ fixupHash: "fixup",
+ expectedTodos: []todo.Todo{
+ {Command: todo.Merge, Commit: "original"},
+ {Command: todo.Fixup, Commit: "fixup"},
+ {Command: todo.Pick, Commit: "other"},
+ },
+ expectedErr: nil,
+ },
  {
  name: "More original hashes than expected",
  todos: []todo.Todo{