amend: allow aborting an `amend --patch` by saving an empty file (issue5925) This patch fixes issue5925 by allow aborting a `hg amend --patch` call by saving an empty patch file. This is similar to aborting of `hg amend` when commit message is empty. Test changes demonstrate the fix.

# utility to setup pythonpath to point into the tested repository

export SRCDIR=`dirname $TESTDIR`
if [ -n "$PYTHONPATH" ]; then
    export HGTEST_ORIG_PYTHONPATH=$PYTHONPATH
    export PYTHONPATH=$SRCDIR:$PYTHONPATH
else
    export PYTHONPATH=$SRCDIR
fi