Document how to get a specific revision when rebaselining.

This is helpful when using the trybots.

R=epoger@google.com

Review URL: https://codereview.chromium.org/25653008

git-svn-id: http://skia.googlecode.com/svn/trunk@11643 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/rebaseline.py b/tools/rebaseline.py
index a1f92cb..275d84e 100755
--- a/tools/rebaseline.py
+++ b/tools/rebaseline.py
@@ -344,7 +344,8 @@
 parser.add_argument('--actuals-base-url',
                     help=('base URL from which to read files containing JSON '
                           'summaries of actual GM results; defaults to '
-                          '%(default)s'),
+                          '%(default)s. To get a specific revision (useful for'
+                          'trybots) replace "svn" with "svn-history/r123".'),
                     default='http://skia-autogen.googlecode.com/svn/gm-actual')
 parser.add_argument('--actuals-filename',
                     help=('filename (within builder-specific subdirectories '