outgoing: merge the code handling --graph with the main one The --graph code had its own copy of the logic. With the previous reorganisation of the code, we can now merge it with the main code, reducing fragile complication. As a side effect, `hg out --graph` now use the right return code when they are nothing outgoing. This explain the change to output in `tests/test-largefiles-misc.t`. Differential Revision: https://phab.mercurial-scm.org/D10383