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.
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions