test-keyword: use regular expressions instead of grepping

tests/test-keyword.t

@@ -331,9 +331,10 b' Cat modified file a'
 
 Diff remaining chunk
 
-  $ hg diff | grep -v 'b/a'
+  $ hg diff
   diff -r d17e03c92c97 a
   --- a/a	Wed Dec 31 23:59:51 1969 -0000
+  \+\+\+ b/a	.*
   @@ -2,3 +2,4 @@
    foo
    do not process $Id:
@@ -474,9 +475,10 b' Test different options of hg kwfiles'
 
 Diff specific revision
 
-  $ hg diff --rev 1 | grep -v 'b/c'
+  $ hg diff --rev 1
   diff -r ef63ca68695b c
   --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  \+\+\+ b/c	.*
   @@ -0,0 +1,3 @@
   +expand $Id$
   +do not process $Id: