rcutorture: Add "git diff" output to testid.txt file

Currently, when running from a git archive, the testid.txt file contains
only the branch name, the output of "git status", and the SHA-1 of
the current HEAD.  This is useful, but does not uniquely identify the
source code that was built.  This commit therefore adds the output of
"git diff HEAD", which means that if two testid.txt files compare equal,
they correspond to exactly the same source code.  Give or take the
possibility of SHA-1 collisions, that is.  ;-)
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/bin/kvm.sh

@@ -296,10 +296,7 @@ if test -d .git
 then
 	git status >> $resdir/$ds/testid.txt
 	git rev-parse HEAD >> $resdir/$ds/testid.txt
-	if ! git diff HEAD > $T/git-diff 2>&1
-	then
-		cp $T/git-diff $resdir/$ds
-	fi
+	git diff HEAD >> $resdir/$ds/testid.txt
 fi
 ___EOF___
 awk < $T/cfgcpu.pack \