rhg: try read [paths] for `--repository` value

hg parses -R and --repository CLI arguments "early" in order to know which local repository to load config from. (Config can then affect whether or how to fall back.)

The value of of those arguments can be not only a filesystem path, but also an alias configured in the [paths] section. This part was missing in rhg and this patch implements that.

The #if no-rhg is still not removed completely as those two [paths] are read from .hg/hgrc of directory in which the command is running. That will be implemented in the next patch.