safehasattr: pass attribute name as string instead of bytes
This is a step toward replacing `util.safehasattr` usage with plain `hasattr`.
The builtin function behave poorly in Python2 but this was fixed in Python3.
These change are done one by one as they tend to have a small odd to trigger
puzzling breackage.
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https://www.mercurial-scm.org/wiki/ContributingChanges
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https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
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273ce12ad8f1` will report some style violations on a very old commit).