bisect: move check_state into the bisect module Now that the function is simpler, we resume our quest to move the logic into the bisect module. In the process, we add basic documentation.

bisect: simplify conditional in 'check_state' Now that extra code about "updating" flag have been removed, we can simplify the condition flow and remove a level.

bisect: remove code about "update-flag" in check_state Now that the flag dedicated to updating the flag are handled earlier, we do not need to handle them in the 'check_state' function.

bisect: rename 'check_code' to match our naming scheme We need to do it early, otherwise 'check-commit' will complain every time we touch it.