messages with the same id in two branches doesn't work when use_name_for_id=true

What steps will reproduce the problem?
1. add a message with the same id in two different conditional branches and set 
'use_name_for_id' to true
2. run grit 'compiler'

What is the expected output? What do you see instead?

Actual:  I got the following error : 

MakeClique
    assert c.GetMessage().GetPresentableContent() == presentable_text

Expected: The message in a branch meeting the condition is processed while 
other branches are ignored. 

What version of the product are you using? On what operating system?

grit-i18n @ r=84 on Linux, but it should be platform-independent. 

Please provide any additional information below.

See https://chromiumcodereview.appspot.com/11338052/#ps1

Original issue reported on code.google.com by jshin@chromium.org on 14 Nov 2012 at 9:15

metasmile / grit-i18n

messages with the same id in two branches doesn't work when use_name_for_id=true #8