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.

mercurial/strip.py

@@ -36,7 +36,7 b' def _findupdatetarget(repo, nodes):'
     currentbranch = repo[None].branch()
 
     if (
-        util.safehasattr(repo, b'mq')
+        util.safehasattr(repo, 'mq')
         and p2 != repo.nullid
         and p2 in [x.node for x in repo.mq.applied]
     ):