obsolete: extract the part of _readmarkers specific to format version 0

If we are to introduce new formats we need to be able call different
functions for different formats. Creating a function for format
version 0 is the first step.

mercurial/obsolete.py

@@ -145,7 +145,10 b' def _readmarkers(data):'
     if diskversion != _fm0version:
         raise util.Abort(_('parsing obsolete marker: unknown version %r')
                          % diskversion)
+    return _fm0readmarkers(data, off)
 
+def _fm0readmarkers(data, off=0):
+    """Read and enumerate markers from raw data in format version 0"""
     # Loop on markers
     l = len(data)
     while off + _fm0fsize <= l: