Do not throw an exception if git cl rebase fails.

Otherwise I get a stack trace and it's annoying as hell.

R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/8201012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@104770 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -1302,8 +1302,7 @@ def CMDrebase(parser, args):
   # git svn dcommit.
   # It's the only command that doesn't use parser at all since we just defer
   # execution to git-svn.
-  subprocess2.check_call(['git', 'svn', 'rebase'] + args)
-  return 0
+  return subprocess2.call(['git', 'svn', 'rebase'] + args)
 
 
 def GetTreeStatus():