test: add squash-merge detection tests for rebase_continue and rebase_skip

Add 3 integration tests covering the remaining HIGH priority gaps (H4-H6)
for squash-merge detection during continue and skip code paths. These tests
verify that is_squashed_merged() correctly detects squash-merged branches
when they are encountered during --continue or --skip, not just during the
initial rebase call.

Author: dashed