bundle2: rename the _canusebundle2 method to _forcebundle1

We rename and invert the logic of the _canusebundle2 utility. The idea here is
that we need to have a way to enforce the use of bundle1 in the tests. The
Mercurial philosophy is to try to use the best method available. Currently that
best method is bundle2, but this might change in the future. Therefore expressing
"do not use bundle2" is a loosy way to say "use bundle 1" and will likely create
issue in the future. As the config option will be explicitly about bundle1, we
rename the function beforehand to align with this. This will make the life of a
future developer working on bundle3 easier.

mercurial/exchange.py

@@ -257,11 +257,11 b' def buildobsmarkerspart(bundler, markers'
         return bundler.newpart('obsmarkers', data=stream)
     return None
 
-def _canusebundle2(op):
-    """return true if a pull/push can use bundle2
+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 (op.repo.ui.configbool('experimental', 'bundle2-exp', True)
+    return not (op.repo.ui.configbool('experimental', 'bundle2-exp', True)
             and op.remote.capable('bundle2'))
 
 
@@ -417,7 +417,7 b' def push(repo, remote, force=False, revs'
         # bundle2 push may receive a reply bundle touching bookmarks or other
         # things requiring the wlock. Take it now to ensure proper ordering.
         maypushback = pushop.ui.configbool('experimental', 'bundle2.pushback')
-        if _canusebundle2(pushop) and maypushback:
+        if (not _forcebundle1(pushop)) and maypushback:
             localwlock = pushop.repo.wlock()
         locallock = pushop.repo.lock()
         pushop.locallocked = True
@@ -442,7 +442,7 b' def push(repo, remote, force=False, revs'
             lock = pushop.remote.lock()
         try:
             _pushdiscovery(pushop)
-            if _canusebundle2(pushop):
+            if not _forcebundle1(pushop):
                 _pushbundle2(pushop)
             _pushchangeset(pushop)
             _pushsyncphase(pushop)
@@ -1099,7 +1099,7 b' class pulloperation(object):'
 
     @util.propertycache
     def canusebundle2(self):
-        return _canusebundle2(self)
+        return not _forcebundle1(self)
 
     @util.propertycache
     def remotebundle2caps(self):