revset: move weight information to predicate
Previously revset weight is hardcoded and cannot be modified. This patch
moves it to predicate so newly registered revsets could define their weight
to properly give static optimization some hint.
Differential Revision: https://phab.mercurial-scm.org/D657
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif