evolve: fix divergence resolution when not merging a descendant In divergence resolution, when we merge other cset with divergent one we pass a `mergeancestor` arg to merge.update() and before this patch we were passing `mergeancestor` as True in every case. But it should be True only when we are merging a descendant onto an ancestor. When mergeancestor is True it does two things: 1) allows the merge if the destination is the same as the parent of the ctx (so we can use graft to copy commits) 2) informs update that the incoming changes are newer than the destination so it doesn't prompt about "remote changed foo which local deleted". So this patch change it to pass `mergeancestor` as True only when it is required. And changes in test file shows that it wasn't prompting either in those cases when it should (acc. to 2nd point) Test written by Pierre-Yves David, based the one updated in 5dbaabfe2c59.

# utility to setup pythonpath to point into the tested repository

export SRCDIR="`dirname $TESTDIR`"
if [ -n "$PYTHONPATH" ]; then
    export HGTEST_ORIG_PYTHONPATH=$PYTHONPATH
    if uname -o | grep -q Msys; then
        export PYTHONPATH="$SRCDIR;$PYTHONPATH"
    else
        export PYTHONPATH=$SRCDIR:$PYTHONPATH
    fi
else
    export PYTHONPATH=$SRCDIR
fi