py3: use .startswith() instead of bytes[0]

Differential Revision: https://phab.mercurial-scm.org/D2040

mercurial/revset.py

@@ -1841,7 +1841,7 b' def _getsortargs(x):'
     keyflags = []
     for k in keys.split():
         fk = k
-        reverse = (k[0] == '-')
+        reverse = (k.startswith('-'))
         if reverse:
             k = k[1:]
         if k not in _sortkeyfuncs and k != 'topo':