bundle2: add a devel option controling bundle version used for exchange

We need an official way to force bundle1 to be used in test. We introduce a new
option 'devel.legacy.exchange' to control this. When specified, this option
will control the list of bundle version Mercurial consider when exchanging with
a peer. Current valid value are 'bundle1' and 'bundle2'.

Using this option in all tests will allow us to remove the
'experimental.bundle2-exp' option. We will simplify the code once the
experimental option is dropped.

mercurial/exchange.py

@@ -260,10 +260,23 b' def buildobsmarkerspart(bundler, markers'
 def _forcebundle1(op):
     """return true if a pull/push must use bundle1
 
-    Feel free to nuke this function when we drop the experimental option"""
-    return not (op.repo.ui.configbool('experimental', 'bundle2-exp', True)
-                and op.remote.capable('bundle2'))
+    This function is used to allow testing of the older bundle version"""
+    ui = op.repo.ui
+    forcebundle1 = False
+    # The goal is this config is to allow developper to choose the bundle
+    # version used during exchanged. This is especially handy during test.
+    # Value is a list of bundle version to be picked from, highest version
+    # should be used.
+    #
+    # developer config: devel.legacy.exchange
+    exchange = ui.configlist('devel', 'legacy.exchange')
+    if not exchange:
+        forcebundle1 = not ui.configbool('experimental', 'bundle2-exp', True)
+        # developer config: devel.legacy.exchange
+    else:
+        forcebundle1 = 'bundle2' not in exchange and 'bundle1' in exchange
 
+    return forcebundle1 or not op.remote.capable('bundle2')
 
 class pushoperation(object):
     """A object that represent a single push operation