py3: make py3 compat.iterbytestr simpler and faster
With Python 3.4.3, timit says 11.9 usec-> 6.44 usec. With Python
3.6.0, timeit says 14.1 usec -> 9.55 usec.
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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
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).