git-gui: lib/index.tcl: handle files with % in the filename properly

git-gui: lib/index.tcl: handle files with % in the filename properly

Steps to reproduce the bug:

 $ mkdir repo && cd repo && git init
 Initialized empty Git repository in .git/
 $ touch 'foo%3Fsuite'
 $ git-gui

Then click on the 'foo%3Fsuite' icon to include it in a changeset, a
popup comes with:
'Error: bad field specifier "F"'

Vincent Danjean noticed the problem and also suggested the fix, reported
through
 http://bugs.debian.org/441167

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>