b4: wrong base-branch in non commit tracking info

Hi,

I'm currently trying a bit with adding a git notes based cover letter strategy. While working on that I found that if the cover letter strategy is not commit, and the user preps a new series with a fork point (-f) the current branch that is checked out is tracked as base-branch, even if current commits do not appear in the fork point.

Didn't yet understand the prep base code sufficent to directly send a patch.

I'm not yet sure how well a notes based cover letter stragety will work and want to test it before sending a patch, but the current WIP branch is in my git[0]

Thanks,
Fiona

[0]: https://cyberchaos.dev/kloenk/b4/-/commits/cover-strategy-notes