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
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors