cmdutil: replace sys.maxint with None as default value in loglimit
Semantically, it is better to use None over any other value when there is
"no value". Using maxint in this context is quite hackish, and is not forward
compatible.
adding foo
% no signatures
% hg sign 0
Signing 0:e63c23eaa88a
% hg sigs
hgtest 0:e63c23eaa88ae77967edcf4ea194d31167c478b0
% hg sigcheck 0
e63c23eaa88a is signed by:
hgtest