push: fix subtle bug in prepush logic
A test case demonstrating the bug has been added.
Contributed by Henrik Stuart and Sune Foldager.
#!/bin/sh
hg init a
echo a > a/a
hg --cwd a ci -A -m a
hg clone a b
echo b > b/b
hg --cwd b ci -A -m b
chmod 100 a/.hg/store
hg --cwd b push ../a
chmod 700 a/.hg/store