py3: conditionalize access to socketserver.ForkingMixIn
This is no longer exported on platforms that don't support forking, as of 3.6.
https://github.com/python/cpython/commit/
aadff9bea61a2fc9f4cf0f213f0ee50fc54d6574
convert: fix a file descriptor leak
test-check-code flagged this after I changed this line for something unrelated.
revlog: catch more specific exception in shortest()
Since revlog._partialmatch() catches RevlogError coming from cext and
re-raises AmbiguousPrefixLookupError, catching RevlogError here seems
less correct.
Differential Revision: https://phab.mercurial-scm.org/D4735