Clamp negative rev numbers at zero.
Prior to this change, trying to run "hg log -r -50:" in a repo with
less than 50 changes caused an error. Now that we clamp at zero: no
more error.
\.elc$
\.orig$
\.rej$
~$
\.so$
\.pyc$
\.swp$
\.prof$
^tests/.*\.err$
^build/
^dist/
^doc/.*\.[0-9](\.(x|ht)ml)?$
^MANIFEST$
^\.pc/
^patches/
^mercurial/__version__.py$