The apparent culprit is the file diff.pro (the PostScript Prologue file for a2ps), which is usually located at /usr/share/a2ps/ps/diff.pro. You can replace your diff.pro file with my modified diff.pro.
The modificarions made are (unified diff):
--- diff.pro-orig 2010-08-14 12:35:00.320960289 +0200 +++ diff.pro 2010-08-14 13:07:18.227961317 +0200 @@ -2,7 +2,7 @@ % PostScript Prologue for a2ps (meant for diffs) % % $Id: diff.pro,v 1.2 1998/04/08 14:54:21 demaille Exp $ -% +% jj: modified to fix pdiff 2010-August-14 % % This file is part of a2ps. @@ -68,11 +68,12 @@ Show } bind def +%% jj: % Removed /k { 0 0 0 FG - false BG - true UL + 0.8 0.8 0.8 true BG + false UL false BX %Face: Keyword Courier bfs Show @@ -83,7 +84,7 @@ 0 0 0 FG 0.9 0.9 0.9 true BG false UL - false BX + true BX %Face: Keyword_strong Courier bfs Show } bind def