Fix the squash-commit idempotency check to resolve in real action runs

[Phlogistique]

has_squash_commit compared the target with a bare branch name like "main".
actions/checkout leaves a detached HEAD and creates no local branches, so
"main" does not resolve and git exits 128, which the if silently reads as
"not up-to-date". The skip branch was therefore dead in CI and only worked
in tests, where the test repos have every branch locally.

This matters on the resume path: the push that finishes conflict resolution
fires a synchronize event while the conflict label is still on. If that
second run loses the race against the label removal, the broken check let it
re-run the whole merge and push a second redundant synthetic merge commit.

Compare against origin/$TARGET_BRANCH instead. With fetch-depth: 0 the
remote-tracking ref always exists, so the check works in production while
staying correct in tests.