obsolete: add a "format.obsstore-version" config option

This option controls what version of the binary format to use when creating a new
obsstore file.

Default is still the old format. No safeguards are currently placed around the
option value, but no clueless users are in danger of harm since it is
undocumented.

mercurial/localrepo.py

@@ -400,7 +400,13 b' class localrepository(object):'
 
     @storecache('obsstore')
     def obsstore(self):
-        store = obsolete.obsstore(self.sopener)
+        # read default format for new obsstore.
+        defaultformat = self.ui.configint('format', 'obsstore-version', None)
+        # rely on obsstore class default when possible.
+        kwargs = {}
+        if defaultformat is not None:
+            defaultformat['defaultformat'] = defaultformat
+        store = obsolete.obsstore(self.sopener, **kwargs)
         if store and not obsolete._enabled:
             # message is rare enough to not be translated
             msg = 'obsolete feature not enabled but %i markers found!\n'

mercurial/obsolete.py

@@ -450,7 +450,7 b' class obsstore(object):'
     # parents: (tuple of nodeid) or None, parents of precursors
     #          None is used when no data has been recorded
 
-    def __init__(self, sopener):
+    def __init__(self, sopener, defaultformat=_fm0version):
         # caches for various obsolescence related cache
         self.caches = {}
         self._all = []
@@ -459,7 +459,7 b' class obsstore(object):'
         self.children = {}
         self.sopener = sopener
         data = sopener.tryread('obsstore')
-        self._version = _fm0version
+        self._version = defaultformat
         if data:
             self._version, markers = _readmarkers(data)
             self._load(markers)