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/commandserver.py

@@ -384,7 +384,7 b' class server:'
         if self.cmsg:
             hellomsg += b'message-encoding: %s\n' % self.cmsg.encoding
         hellomsg += b'pid: %d' % procutil.getpid()
-        if util.safehasattr(os, b'getpgid'):
+        if util.safehasattr(os, 'getpgid'):
             hellomsg += b'\n'
             hellomsg += b'pgid: %d' % os.getpgid(0)