paths: do not process default-push as pushurl of default path (
issue5000)
It didn't work because "default-push" and "default" are independent named
items. Without this patch, "hg push default" would push to "default-push"
because paths["default"].pushloc was overwritten by "default-push".
Also, we shouldn't ban a user from doing "hg push default-push" so long as
"default-push" item is defined, not "default:pushurl". Otherwise, he would
be confused by missing "default-push" path.
Tests are included in a patch for stable branch.