Add Myers' algorithm to generate diff scripts

Myers' algorithm is the standard way to generate diff scripts in
an efficient manner (especially memory-wise).

The source contains extensive documentation about the principal
ideas of the algorithm.

Bug: 291083
Eclipse-CQ: 3559
Change-Id: Id873e7c2c204692cb4a1a05d24b733154b9ed49c
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>