shallowutil: fsdecode the bytes group name before passing to os

Differential Revision: https://phab.mercurial-scm.org/D5608

hgext/remotefilelog/shallowutil.py

@@ -420,7 +420,7 b' def readpathlist(stream):'
 
 def getgid(groupname):
     try:
-        gid = grp.getgrnam(groupname).gr_gid
+        gid = grp.getgrnam(pycompat.fsdecode(groupname)).gr_gid
         return gid
     except KeyError:
         return None