Enhance the file filtering capabilities.
We now allow filtering through either pipes or pairs of temporary
files. The latter appear to be mandatory for use on Windows.
%% no merges expected
%% merge of b expected
merging for b
merging b
%%
Contents of b should be "this is file b1"
This is file b1
%% merge expected!
merging for b
merging b
%% merge of b expected
merging for b
merging b