rebaseline_server: add actual_repo and actual_revision command line options

So, if you want to see divergences from expectations as of https://code.google.com/p/skia-autogen/source/detail?r=29365 , specify --actuals-revision 29365

This is also handy for testing rebaseline_server performance improvements against a static set of results.

BUG=skia:2248
NOTREECHECKS=True
NOTRY=True
R=borenet@google.com, bsalomon@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/193023002

git-svn-id: http://skia.googlecode.com/svn/trunk@13726 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/svn.py b/tools/svn.py
index 0c1ec4f..7927258 100644
--- a/tools/svn.py
+++ b/tools/svn.py
@@ -95,14 +95,15 @@
         """
         return self._RunCommand([SVN, 'checkout', url, path])
 
-    def Update(self, path):
+    def Update(self, path, revision='HEAD'):
         """Update the working copy.
         Returns stdout as a single string.
 
         @param path path (within self._directory) within which to run
-        "svn update"
+          "svn update"
+        @param revision revision to update to
         """
-        return self._RunCommand([SVN, 'update', path])
+        return self._RunCommand([SVN, 'update', path, '--revision', revision])
 
     def ListSubdirs(self, url):
         """Returns a list of all subdirectories (not files) within a given SVN