log: add an extension hook-point in changeset_printer

Extensions sometimes wants to add other information in the default log output
format (when no templating is used).

Add an empty function named '_exthook' for easing the extension life.
Extensions will be able to wrap this function and collaborate to display
additional information.

Exthook is called after displaying troubles and just before displaying the
files, extra and description.

Add a new test file to test it and not pollute other test files.

tests/test-log-exthook.t

@@ -0,0 +1,56 b''
+Test hg log changeset printer external hook
+-------------------------------------------
+
+  $ cat > $TESTTMP/logexthook.py <<EOF
+  > from mercurial import repair, commands
+  > from mercurial import cmdutil
+  > def rot13description(self, ctx):
+  >     summary = "summary".encode('rot13')
+  >     description = ctx.description().strip().splitlines()[0].encode('rot13')
+  >     self.ui.write("%s:     %s\n" % (summary, description))
+  > def reposetup(ui, repo):
+  >     cmdutil.changeset_printer._exthook = rot13description
+  > EOF
+
+Prepare the repository
+
+  $ hg init empty
+  $ cd empty
+  $ touch ROOT
+  $ hg commit -A -m "Root" ROOT
+
+  $ touch a b c
+  $ hg commit -A -m "Add A, B, C" a b c
+
+Check the log
+
+  $ hg log --config extensions.t=$TESTTMP/logexthook.py
+  changeset:   1:70fc82b23320
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  fhzznel:     Nqq N, O, P
+  summary:     Add A, B, C
+  
+  changeset:   0:b00443a54871
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  fhzznel:     Ebbg
+  summary:     Root
+  
+Check that exthook is working with graph log too
+
+  $ hg log -G --config extensions.t=$TESTTMP/logexthook.py
+  @  changeset:   1:70fc82b23320
+  |  tag:         tip
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  fhzznel:     Nqq N, O, P
+  |  summary:     Add A, B, C
+  |
+  o  changeset:   0:b00443a54871
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     fhzznel:     Ebbg
+     summary:     Root
+  

mercurial/cmdutil.py

@@ -1411,6 +1411,8 b' class changeset_printer(object):'
             self.ui.write(_("trouble:     %s\n") % ', '.join(ctx.troubles()),
                           label='log.trouble')
 
+        self._exthook(ctx)
+
         if self.ui.debugflag:
             files = ctx.p1().status(ctx)[:3]
             for key, value in zip([# i18n: column positioning for "hg log"
@@ -1457,6 +1459,11 b' class changeset_printer(object):'
 
         self.showpatch(ctx, matchfn)
 
+    def _exthook(self, ctx):
+        '''empty method used by extension as a hook point
+        '''
+        pass
+
     def showpatch(self, ctx, matchfn):
         if not matchfn:
             matchfn = self.matchfn