obsstore: cache size computation for fm1 node

We have two different types of node type (sha1 and sha256, only sha1 is used
now) and therefor different sizes for them. We now compute the value once
instead of redoing the computation every loop. This has no visible performance
impact.

mercurial/obsolete.py

@@ -276,6 +276,8 b' def _fm0decodemeta(data):'
 _fm1fixed = '>IdhHBBB20s'
 _fm1nodesha1 = '20s'
 _fm1nodesha256 = '32s'
+_fm1nodesha1size = _calcsize(_fm1nodesha1)
+_fm1nodesha256size = _calcsize(_fm1nodesha256)
 _fm1fsize = _calcsize(_fm1fixed)
 _fm1parentnone = 3
 _fm1parentshift = 14
@@ -298,9 +300,10 b' def _fm1readmarkers(data, off=0):'
         # build the date tuple (upgrade tz minutes to seconds)
         date = (seconds, tz * 60)
         _fm1node = _fm1nodesha1
+        fnodesize = _fm1nodesha1size
         if flags & usingsha256:
             _fm1node = _fm1nodesha256
-        fnodesize = _calcsize(_fm1node)
+            fnodesize = _fm1nodesha256size
         # read replacement
         sucs = ()
         if numsuc: