this moves the actual diff algorithm into a different package and then provides hooks so it can be easily replaced with an alternate algorithm.
Change-Id: Ia0359f58878493599ea0e0fda8920f21100e16f1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/190898
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>