Fix behaviour of commit.
It's now a fatal error if the option to --logfile isn't readable.
Ditto if both --message and --logfile are specified.
%% 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