Drop was passing a rev number to cleanupnodes, which isn't supported. New
versions of hg throw an exception when this happens, so this was caught by the
tests.
Summary: Adding a drop extension which drops specified commit from the stack.
Test Plan: Unit tests which are part of this diff and manual testing.
Reviewers: durham, gshanbhag, #mercurial, kulshrax
Reviewed By: kulshrax
Subscribers: mitrandir, quark, #mercurial, phillco, avig, medson, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D5346014
Tasks: 19602078
Signature: t1:5346014:1500438430:b38acce97940fdeb7103ae70844cce71fdd79698
