uname -s is used to determine the differences between the command line
utilities like stat or awk in linux and darwin. However, in nix, guilt
will be using the nix versions of this programs, not the ones
installed in the system.  Therefore, guilt should use the command-line
parameters that the linux forms of these commands expect, even if it
is being run on Darwin.

diff --git a/guilt b/guilt
index bf50343..cfc9332 100755
--- a/guilt
+++ b/guilt
@@ -986,7 +986,7 @@ guards_file="$GUILT_DIR/$branch/guards"
 pager="more"
 [ ! -z "$PAGER" ] && pager="$PAGER"
 
-UNAME_S=`uname -s`
+UNAME_S="Linux"
 
 if [ -r "$GUILT_PATH/os.$UNAME_S" ]; then
 	. "$GUILT_PATH/os.$UNAME_S"