phabricator: warn if unable to amend, instead of aborting after posting
There was a divergence in behavior here between obsolete and strip based
amending. I first noticed the abort when testing outside of the test harness,
but then had trouble recreating it here after reverting the code changes. It
turns out, strip based amend was successfully amending the public commit after
it was posted! It looks like the protection is in the `commit --amend` command,
not in the underlying code that it calls.
I considered doing a preflight check and aborting. But the locks are only
acquired at the end, if amending, and this is too large a section of code to be
wrapped in a maybe-it's-held-or-not context manager for my tastes.
Additionally, some people do post-push reviews, and amending is the default
behavior, so they shouldn't see a misleading error message.
The lack of a 'Differential Revision' entry in the commit message breaks a
{phabreview} test, so it had to be partially conditionalized.
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