Fix hg push and hg push -r sometimes creating new heads without --force. Fixing issue179. The algorithm checks if there not more new heads on the remote side than heads which become non-heads due to getting children. Pushing this repo: m /\ 3 3a| |/ / 2 2a |/ 1 to a repo only having 1, 2 and 3 didn't abort requiring --force before. Added test cases for this and some doc strings for used methods.

#!/bin/sh

hg init
echo foo > a
hg add a
hg commit -m "1" -d "1000000 0"
hg verify
chmod -r .hg/data/a.d
hg verify 2>/dev/null || echo verify failed
chmod +r .hg/data/a.d
hg verify 2>/dev/null || echo verify failed
chmod -w .hg/data/a.d
echo barber > a
hg commit -m "2" -d "1000000 0" 2>/dev/null || echo commit failed