Two machines filing issues in the same hour both read main correctly and
both took "the next free number". main lags every open pull request —
seven that evening — so they collided twice. The second collision
reached main with records, cycle and index all reporting success.
cycle.py now refuses a tree where two issue folders share a leading
number, and names both. Proven by adding a duplicate and watching it
fail. The colliding records become 153 and 154, renumbered in the branch
that lands last, because renumbering a branch whose author is still
pushing only moves the race.
The check catches a collision; it does not prevent one. Taking a number
still means reading the open pull requests as well as main — issue 155
says so.
jschoubben