commands: mention diff -c

[timeless]

This patch is more for discussion than committing.

Most options take no arguments, -c is like -r, and it's in fact
mutually exclusive with it, but entirely undocumented.

# HG changeset patch
# User timeless <timeless at mozdev.org>
# Date 1266708077 -7200
# Node ID 01e33e2fd61eb1f1a0da8ac54278ba80f3638251
# Parent  261cc6b0f15c7e83dcff7dd7d5f3607c098c29bd
commands: mention diff -c

diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1114,6 +1114,9 @@ def diff(ui, repo, *pats, **opts):
     revisions are specified, the working directory files are compared
     to its parent.

+    Alternatively you can specify -c with a revision to use as a
+    baseline.
+
     Without the -a/--text option, diff will avoid generating diffs of
     files it detects as binary. With -a, diff will generate a diff
     anyway, probably with undesirable results.
@@ -3552,7 +3555,7 @@ table = {
          [('r', 'rev', [], _('revision')),
           ('c', 'change', '', _('change made by revision'))
          ] + diffopts + diffopts2 + walkopts,
-         _('[OPTION]... [-r REV1 [-r REV2]] [FILE]...')),
+         _('[OPTION]... ([-c REV] | [-r REV1 [-r REV2]]) [FILE]...')),
     "^export":
         (export,
          [('o', 'output', '', _('print output to file with formatted name')),