debugcommands: introduce standalone module for debug commands

commands.py is our largest .py file by nearly 2x. Debug commands live
in a world of their own. So let's extract them to their own module.

We start with "debugancestor."

We currently reuse the commands table with commands.py and have a hack
in dispatch.py for loading debugcommands.py. In the future, we could
potentially use a separate commands table and avoid the import of
debugcommands.py.

mercurial/commands.py

@@ -1867,25 +1867,6 @@ def copy(ui, repo, *pats, **opts):
     with repo.wlock(False):
         return cmdutil.copy(ui, repo, pats, opts)
 
-@command('debugancestor', [], _('[INDEX] REV1 REV2'), optionalrepo=True)
-def debugancestor(ui, repo, *args):
-    """find the ancestor revision of two revisions in a given index"""
-    if len(args) == 3:
-        index, rev1, rev2 = args
-        r = revlog.revlog(scmutil.opener(os.getcwd(), audit=False), index)
-        lookup = r.lookup
-    elif len(args) == 2:
-        if not repo:
-            raise error.Abort(_("there is no Mercurial repository here "
-                               "(.hg not found)"))
-        rev1, rev2 = args
-        r = repo.changelog
-        lookup = repo.lookup
-    else:
-        raise error.Abort(_('either two or three arguments required'))
-    a = r.ancestor(lookup(rev1), lookup(rev2))
-    ui.write("%d:%s\n" % (r.rev(a), hex(a)))
-
 @command('debugbuilddag',
     [('m', 'mergeable-file', None, _('add single file mergeable changes')),
     ('o', 'overwritten-file', None, _('add single file all revs overwrite')),

mercurial/debugcommands.py

@@ -0,0 +1,42 @@
+# debugcommands.py - command processing for debug* commands
+#
+# Copyright 2005-2016 Matt Mackall <mpm@selenic.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import os
+
+from .i18n import _
+from . import (
+    cmdutil,
+    commands,
+    error,
+    revlog,
+    scmutil,
+)
+
+# We reuse the command table from commands because it is easier than
+# teaching dispatch about multiple tables.
+command = cmdutil.command(commands.table)
+
+@command('debugancestor', [], _('[INDEX] REV1 REV2'), optionalrepo=True)
+def debugancestor(ui, repo, *args):
+    """find the ancestor revision of two revisions in a given index"""
+    if len(args) == 3:
+        index, rev1, rev2 = args
+        r = revlog.revlog(scmutil.opener(os.getcwd(), audit=False), index)
+        lookup = r.lookup
+    elif len(args) == 2:
+        if not repo:
+            raise error.Abort(_('there is no Mercurial repository here '
+                                '(.hg not found)'))
+        rev1, rev2 = args
+        r = repo.changelog
+        lookup = repo.lookup
+    else:
+        raise error.Abort(_('either two or three arguments required'))
+    a = r.ancestor(lookup(rev1), lookup(rev2))
+    ui.write('%d:%s\n' % (r.rev(a), hex(a)))

mercurial/dispatch.py

@@ -26,6 +26,7 @@ from .i18n import _
 from . import (
     cmdutil,
     commands,
+    debugcommands,
     demandimport,
     encoding,
     error,
@@ -768,6 +769,10 @@ def _dispatch(req):
 
     # (reposetup is handled in hg.repository)
 
+    # Side-effect of accessing is debugcommands module is guaranteed to be
+    # imported and commands.table is populated.
+    debugcommands.command
+
     addaliases(lui, commands.table)
 
     # All aliases and commands are completely defined, now.