The --root option from "git diff-tree" won't do nothing
when is given to commands like git-whatchanged or git-log,
because those always print the initial commit by default.
This fixes the tutorial explaining the function of the
log.showroot configuration variable.
Signed-off-by: Carlos Rica <jasampler@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>