Pressing `b` on a conflict is meant to pick both sides. With the diff3
conflict style, git also renders the common ancestor between the two
sides, and `b` currently keeps that ancestor section too, which is
wrong: the common base is neither side of the merge and must not end up
in the resolved file.
This test captures the current (buggy) behaviour so the follow-up fix
has a clear before/after.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
d79f6c0b38