Allow callers to pass in the dirstate lock in most localrepo.py funcs.
This makes it possible to take the lock once and commit a large number of
patches, without having to read and write the dirstate for each patch.
%% no merges expected
%% merge should fail
abort: 'b' already exists in the working dir and differs from remote
%% merge of b expected
merging for b
merging b
%%
Contents of b should be "this is file b1"
This is file b1
%% merge fails
abort: outstanding uncommited changes
%% merge expected!
merging for b
merging b
%% merge of b should fail
abort: outstanding uncommited changes
%% merge of b expected
merging for b
merging b