bundle2: add a `obsmarkersversion` function to extract supported version

Right next to the function that encodes the supported versions in
capabilities we add a function that decodes the versions out of capabilities.
This is going to be useful to know what formats can be used for exchange.

mercurial/bundle2.py

@@ -146,6 +146,7 b' import util'
 import struct
 import urllib
 import string
+import obsolete
 import pushkey
 
 import changegroup, error
@@ -800,6 +801,12 b' def bundle2caps(remote):'
     capsblob = urllib.unquote(remote.capable('bundle2-exp'))
     return decodecaps(capsblob)
 
+def obsmarkersversion(caps):
+    """extract the list of supported obsmarkers versions from a bundle2caps dict
+    """
+    obscaps = caps.get('b2x:obsmarkers', ())
+    return [int(c[1:]) for c in obscaps if c.startswith('V')]
+
 @parthandler('b2x:changegroup')
 def handlechangegroup(op, inpart):
     """apply a changegroup part on the repo