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/mdiff.py

@@ -213,7 +213,7 b' def chooseblocksfunc(opts=None):'
     if (
         opts is None
         or not opts.xdiff
-        or not util.safehasattr(bdiff, b'xdiffblocks')
+        or not util.safehasattr(bdiff, 'xdiffblocks')
     ):
         return bdiff.blocks
     else: