obsolete: introduce an `_enabled` switch to disable the feature by default

Obsolete markers wide-usage and propagation should be avoided by default until
the obsolete feature is more mature.

This changeset introduce the `_enable` variable and prevent the creation of
obsolete marker if the feature is set to `False` (the default).

More limitation comes in followup changesets.

mercurial/obsolete.py

@@ -58,7 +58,9 b' from i18n import _'
 _pack = struct.pack
 _unpack = struct.unpack
 
-
+# the obsolete feature is not mature enought to be enabled by default.
+# you have to rely on third party extension extension to enable this.
+_enabled = False
 
 # data used for parsing and writing
 _fmversion = 0
@@ -194,6 +196,8 b' class obsstore(object):'
 
         Take care of filtering duplicate.
         Return the number of new marker."""
+        if not _enabled:
+            raise util.Abort('obsolete feature is not enabled on this repo')
         new = [m for m in markers if m not in self._all]
         if new:
             f = self.sopener('obsstore', 'ab')

tests/test-obsolete-changeset-exchange.t

@@ -1,6 +1,13 b''
 Test changesets filtering during exchanges (some tests are still in
 test-obsolete.t)
 
+  $ cat > obs.py << EOF
+  > import mercurial.obsolete
+  > mercurial.obsolete._enabled = True
+  > EOF
+  $ echo '[extensions]' >> $HGRCPATH
+  $ echo "obs=${TESTTMP}/obs.py" >> $HGRCPATH
+
 Push does corrupt remote
 ------------------------
 

tests/test-obsolete.t

@@ -27,10 +27,25 b''
 
   $ hg init tmpa
   $ cd tmpa
+  $ mkcommit kill_me
+
+Checking that the feature is properly disabled
+
+  $ hg debugobsolete -d '0 0' `getid kill_me` -u babar
+  abort: obsolete feature is not enabled on this repo
+  [255]
+
+Enabling it
+
+  $ cat > ../obs.py << EOF
+  > import mercurial.obsolete
+  > mercurial.obsolete._enabled = True
+  > EOF
+  $ echo '[extensions]' >> $HGRCPATH
+  $ echo "obs=${TESTTMP}/obs.py" >> $HGRCPATH
 
 Killing a single changeset without replacement
 
-  $ mkcommit kill_me
   $ hg debugobsolete 0
   abort: changeset references must be full hexadecimal node identifiers
   [255]