phabricator: avoid creating unstable children within the review stack The instability occurred when rebasing something that has already been submitted onto something that hasn't, and then resubmitting the stack. Or as the test shows, just resubmitting and including something earlier that wasn't previously submitted. There's a general case here where any children (not just the ones in the range of commits posted for review) should be re-stabilized. But handling the selected commits here will cause the `local:commit` node values that are tracked on Phabricator to be properly kept in sync. Differential Revision: https://phab.mercurial-scm.org/D8436