Add ui.strict config item.
This turns off command matching by unambiguous prefix, and is a good
thing if you want to write future-proof scripts.
This fixes issue 314.
% init
% commit
adding a
% annotate -c
8435f90966e4: a
% annotate -d
Thu Jan 01 00:00:01 1970 +0000: a
% annotate -n
0: a
% annotate -u
nobody: a
% annotate -cdnu
nobody 0 8435f90966e4 Thu Jan 01 00:00:01 1970 +0000: a