effectflag: detect when diff changed
Store in effect flag when the diff changed between the predecessor and
its successors.
Comparing the diff is not easy because we do not want to incorrectly detect a
effectflag: detect when meta changed
Store in effect flag when the meta changed between the predecessor and its
successors. We blacklisted some known meta that would always changed when
another flag change. For example rebase would always add a meta rebase-source
while the effect flag parents will already detect this situation.
It can happens with various hg commands.
Differential Revision: https://phab.mercurial-scm.org/D540