When diffing against an old revision, handle the case if a file didn't exists.

Generate a fake diff instance of crashing.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/6721030

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@79290 0039d316-1c4b-4281-b951-d872f2087c98

scm.py

@@ -733,7 +733,13 @@ class SVN(object):
     else:
       if info.get("Node Kind") != "directory":
         # Normal simple case.
-        data = SVN.Capture(command)
+        try:
+          data = SVN.Capture(command)
+        except gclient_utils.CheckCallError:
+          if revision:
+            data = GenFakeDiff(filename)
+          else:
+            raise
       # Otherwise silently ignore directories.
     return data