Find fixup commit: show commit(s), even if already merged

### Is your feature request related to a problem? Please describe.

The scenario: Find a bug, fix it. I wonder where it was introduced? `ctrl-f` to fixup and oops, the commit is already merged.

Which commit though? I still want to know, and the error dialog knows it but won't tell me!

### Describe the solution you'd like

Show the commit that introduced the change in the dialog. If it's ambiguous or any other complications come up no need to bother with it IMO, just the common case of one already merged commit found covers 99%.

Basically just add the hash(es) to this error here: https://github.com/jesseduffield/lazygit/blob/7299b224efda4fccea7e62e9c595d2c6b4461639/pkg/gui/controllers/helpers/fixup_helper.go#L110

The comment above says it's not worth reporting (citation needed^^), but I think it is because 100% of the time when that error comes up, the next step is `git blame` to find out the exact information that caused the error in the first place.