effectflag: detect when description changed
Store in effect flag when the description changed between the predecessor and
its successors.
It can happens with "hg commit --amend -e", "hg amend -e" or "histedit".
Differential Revision: https://phab.mercurial-scm.org/D535
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).