[feature request] Support additional source control instructions

[andy31415]

The instructions currently hardcode the use of git diff -U5 --merge-base origin/HEAD. I use jj which is also quite git compatible, however I have to add extra instructions in my GEMINI.md to use it.

it would be nice for the code review to auto-detect. What I am using on my side is jj diff --from 'fork_point(@ | master@origin) --git --context 5 (but I can also imagine maybe master@origin is not appropriate for all projects so this may need some adjustments)

[mctavish]

This absolutely makes sense as a request. I'm happy that you have a workaround for the time being.

I see two aspects of this:

1. Support source control other than git
2. Support diffs other than --merge-base origin/HEAD

[andy31415]

For others in case you just want a copy & paste, this is the relevant excerpt from my GEMINI.md file:

## Source control

I prefer to use jj as a version control system instead of git.
If a `.jj` folder exists top level, assume I use jj instead of git.

Example useful commands:

- show the diff from master (e.g. for code reviews):
  - `jj diff --from master` to just diff changes from master

For code reviews, you can use this command:
`jj diff --from 'fork_point(@ | master@origin) --git --context 5`