Disabling merge in tweakdefaults

Summary: Disabling merge in tweakdefaults to replace the hook

Test Plan: enable, disabled, disabled and meaasge modified

Reviewers: #sourcecontrol, rmcelroy

Differential Revision: https://phabricator.fb.com/D2515407

Tasks: 8593074

tests/test-tweakdefaults.t

@@ -233,6 +233,18 @@ Test tweaked branch command
   $ hg branches --config tweakdefaults.branchesmessage='testing'| head -n0
   testing
 
+Test tweaked merge command
+  $ hg merge | head -n1
+  abort: no matching bookmark to merge - please merge with an explicit rev or bookmark
+  (run 'hg heads' to see all heads)
+
+  $ hg merge --config tweakdefaults.allowmerge=false
+  abort: merging is not supported for this repository -- use rebase instead
+  [255]
+
+  $ hg merge --config tweakdefaults.mergemessage='testing' --config tweakdefaults.allowmerge=false
+  abort: testing
+  [255]
 
 Test that status is default relative
   $ mkdir foo

tweakdefaults.py

@@ -56,6 +56,8 @@ def extsetup(ui):
     options.append(('', 'new', None, _('allow branch creation')))
     wrapcommand(commands.table, 'branches', branchescmd)
 
+    wrapcommand(commands.table, 'merge', mergecmd)
+
     entry = wrapcommand(commands.table, 'status', statuscmd)
     options = entry[1]
     options.append(
@@ -252,6 +254,17 @@ def branchescmd(orig, ui, repo, active, closed, **opts):
         ui.warn(message + '\n')
     return orig(ui, repo, active, closed, **opts)
 
+def mergecmd(orig, ui, repo, node=None, **opts):
+    """
+    Allowing to disable merges
+    """
+    message = ui.config('tweakdefaults', 'mergemessage', 
+        _('merging is not supported for this repository -- use rebase instead'))
+    if ui.configbool('tweakdefaults','allowmerge', True):
+        return orig(ui, repo, node, **opts)
+    else:
+        raise util.Abort(message)
+
 def statuscmd(orig, ui, repo, *pats, **opts):
     """
     Make status relative by default for interactive usage