update: do not pass in user revspec as default destination (issue6044)
When the revsingle() was introduced at 61c0df2b089a, it couldn't handle
revspec=0 (not '0') properly. That's probably why the default was set to
This is technically BC since "hg update ''" was identical to "hg update '.'"
whereas "hg update -r ''" is "hg update", but I believe that's a bug given
no test fails with this change.