rcutorture: Note diffs from git commits

The current scripting only keeps track of the git SHA-1 of the current
HEAD.  This can cause confusion in cases where testing ran in a git
tree where changes had not yet been checked in.  This commit therefore
also records the output of "git diff HEAD" to provide the information
needed to reconstruct the source tree that was tested.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>