test-ssh: show that stdio redirection doesn't work with chg
Running tests with --chg doesn't mean all hg invocations are replaced by
chg. This patch explicitly adds the test for "chg serve --stdio", which does
weird stdio dance.
revset: add tests of heads(commonancestors(..))
The second-to-last one shows the same bug as commonancestors(..): the
result should be 8, not 9.
Differential Revision: https://phab.mercurial-scm.org/D4741