[BUG] git-p4 can't cope with dollars in filenames

Hi!

If you have a dollar in a filename and attempt to submit it to Perforce 
via git-p4 it fails.

e.g. this sequence:

% touch foo\$bar.c
% git commit -m 'whatever' foo\$bar.c
% git-p4 submit

....
....
Submit aborted -- fix problems then use 'p4 submit -c 3'.

The problem is all the places that use the git-p4 read_pipe() function, 
which is a wrapper around os.popen(). popen() does shell expansion which 
does hilarious things to files with dollars in their name.

You can have hours of fun and entertainment with this. Adding a file 
called foo\`fortune\`.h is especially amusing.

I'm going to try to come up with a fix for this.

Luke