Check git config for unsupported options before running

[mattrq]

Git branchless does not support all the options that git supports e.g.(core.splitIndex and extensions.worktreeConfig). However, these can cause git branchless to fail during running (but not always at the start).

For known options incompatible with git branchless, I suggest a check and a clear warning (or prompt to fix) when any branchless command runs (assuming that they are quick to do).

Many of these are optimisations for mono-repos that branchless help with in a different way.

I also think this will help the adoption of git branchless as it will reduce the barrier to entry/perceived flakiness.

[arxanas]

Hi @mattrq, thanks for the feedback! I opened #543 to warn about extensions.worktreeConfig. I tried a few operations with core.splitIndex set, but they didn't fail. Can you give an example of some situations where it causes an issue?