Thank you for the feedback. We are trying to apply all of your suggestions, but we would prefer to rebase the history before doing some of them (like renaming variables).
About the BISECT_OLDNEWMODE file: The current implementation changes almost nothing to revision.c. We thought it was better, even if it needs a new file. The code for bisect uses BISECT_TERMS because 3 states are possible: 'bad/good mode', 'old/new mode', or 'no bisection started' (if BISECT_TERMS doesn't exist). But the other files (like revision.c) don't need all these informations, so we thought it would be good to check if a file exists instead of reusing BISECT_TERMS, which would require reading its content.