This adds unified diff handling to the diff package.
It also adds a lot more testing and also verifies the unified
diff tests against the diff program if you run the tests with
the flag -verify-diff
This functionality is needed for some of the command verbs.
Change-Id: I817438fd25c0b16f3f31578f51a886944e74a948
Reviewed-on: https://go-review.googlesource.com/c/tools/+/171024
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
