git-gui: Ensure .git/info/exclude is honored in Cygwin workdirs

git-gui: Ensure .git/info/exclude is honored in Cygwin workdirs

If we are using Cygwin and the git repository is actually a
workdir (by way of git-new-workdir) but this Tcl process is
a native Tcl/Tk and not the Cygwin Tcl/Tk then we are unable
to traverse the .git/info path as it is a Cygwin symlink and
not a standard Windows directory.

So we actually need to start a Cygwin process that can do the
path translation for us and let it test for .git/info/exclude
so we know if we can include that file in our git-ls-files or
not.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>