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 a
cd a
echo a > foo
hg add foo
hg commit -m 1 -d "1000000 0"
rm foo
hg remove foo
hg commit -m 2 -d "1000000 0"
hg export 0
hg export 1
hg log -p -r 0
hg log -p -r 1

cd ..
hg clone a b