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

mkdir a
cd a
hg init
echo foo > b
hg add b
hg ci -m "b" -d "1000000 0"

chmod -w .hg/store

cd ..

hg clone a b

chmod +w a/.hg/store # let test clean up

cd b
hg verify