view CONTRIBUTING @ 44719:c482e2fe444c

phabricator: prevent posting obsolete commits

I don't see why this would be useful in the first place.  But I had a coworker
submit a single commit that was not a branch head, and the result was to orphan
its child and keep the original commit visible.  He then did up arrow + Enter,
and it happily created a new review (since the URL isn't amended into the
original commit specified on the command line) and a new successor, resulting in
a local divergence.  I'd like to fix the issue with creating orphans, but this
is simple enough to prevent on its own.

Differential Revision: https://phab.mercurial-scm.org/D8437

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