merge: fix spurious merges for copies in linear updates We make better use of contexts to accurately identify copies that don't need merges. Add a simple test and update other tests.

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a       
echo a > a 
hg commit -Ama 

hg parents -r 0 doesnotexist
true