Better context variables for custom commands

**Is your feature request related to a problem? Please describe.**
I want to write custom commands to use `git difftool`, but `SelectedSubCommit` and `SelectedCommitFile` don't give me enough to work with.

**Describe the solution you'd like**
I'd like these objects to have pointers to their containers. For example `SelectedCommitFile.Commit` or `SelectedSubCommit.Remote`

**Describe alternatives you've considered**
Maybe an overall better solution is to add a command to lazygit that allows you to launch `git difftool` for the current diff view. This would be even more powerful as it would be able to use lazygit's great diff features.

**Additional context**
None