i implemented optimizations that seemed to work well. the big remaining
   one is the enumeration of commits at the start. i started debugging it,
   but ran into issues again. i might need either a local build of gdb or
   some more carefulness or something:
   Breakpoint 1, repo_commits::repo_commits (path=<optimized out>,
   this=<optimized out>) at process2.cpp:75
   75              auto revwalk = repository.create_revwalk();
   (gdb) s
   main (argc=2, argv=0x7fffffffe2e8) at process2.cpp:787
   787                         possible_conflicts =
   repository.merge_commits(commit.parent(0), commit.parent(1));
   the source line it's on keeps jumping around, epecially in the area i
   want to debug. most recently it seemed gdb typed a bunch of things in
   on its own, completely changing where it was and what it was doing.
   meanwhile my system has begun responding at a snails pace, this email
   taking minutes to slowly process each letter.