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.
%% Should show unknown
? unknown
removing b
%% Should show unknown and b removed
R b
? unknown
%% Should show a and unknown
a
unknown