also improve the actual verification of the unified diff
Change-Id: I9c23c24e1fc8571cce2a7879463659ec7069fe99
Reviewed-on: https://go-review.googlesource.com/c/tools/+/199738
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
