tests/test-revlog-group-emptyiter

update: add comments and test cases for updating across branches

Add comment to merge.py:update() showing various cases of 'hg update': to a
descendant, crossing named branches, and crossing branches within a named
branch; with no option, -c or -C; with or without uncommitted changes; and
with or without a specific revision. Add tests for all of these cases.

#! /bin/sh

# issue 1678

echo "# -- setting up base repo"
hg init a
cd a
touch a
hg ci -Am a
cd ..

echo "# -- cloning base repo"
hg clone a b
cd b

echo "# -- setting up cset to push"
hg up null
touch a
hg ci -Am b # different msg so we get a clog new entry

echo "# -- pushing"
hg push -f ../a