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: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1381,6 +1381,7 @@ proc mapdesc {state path} {
 }
 
 proc escape_path {path} {
+	regsub -all {\\} $path "\\\\" path
 	regsub -all "\n" $path "\\n" path
 	return $path
 }