diff: Default arguments to HEAD, working directory

Similar to C Git, default our difference when no trees are given
to us to something that makes a tiny bit of sense to the human.

We also now support the --cached flag, and have its meaning work the
same way as C Git.

Change-Id: I2f19dad4e018404e280ea3e95ebd448a4b667f59
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>