vimgrep over arglist doesn't work on Windows

[GoogleCodeExporter]

What steps will reproduce the problem?
0. Use vim on Windows 

1. Have a few files in your arglist with full paths. 
E.g. 
\Work\test.py \Users\User\another.py

2. do the following command
:vimgrep /def/ ##

What is the expected output? What do you see instead?
vimgrep should find all mentions of "def" in the two files in my arglist. 
Instead I get a long pause and finally an error complaining about a bad path 
Cannot open file "\\Work\\test.py"

What version of the product are you using? On what operating system?
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Aug 25 2014 04:20:13)
MS-Windows 32-bit console version 
running on Win 8.1 x64

Please provide any additional information below.
The problem is presumably that when ## is expanded it doubles the backslash 
which isn't supported for vimgrep

Original issue reported on code.google.com by jonthoro...@gmail.com on 6 May 2015 at 1:43