git-gui: Replace \ with \\ when showing paths.

git-gui: Replace \ with \\ when showing paths.

We already replace \n with \\n so that Tk widgets don't start a new
display line with part of a file path which is just unlucky enough
to contain an LF.  But then its confusing to read a path whose name
actually contains \n as literal characters.  Escaping \ to \\ would
make that case display as \\n, clarifying the output.

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