obsolete: reuse _fm1metapair for computing _fm1metapairsize

It's evaluated at import time, so it seems silly to not reuse the
constant name.

mercurial/obsolete.py

@@ -314,7 +314,7 b' def _fm0decodemeta(data):'
 _fm1parentshift = 14
 _fm1parentmask = (_fm1parentnone << _fm1parentshift)
 _fm1metapair = 'BB'
-_fm1metapairsize = _calcsize('BB')
+_fm1metapairsize = _calcsize(_fm1metapair)
 
 def _fm1purereadmarkers(data, off, stop):
     # make some global constants local for performance