internal/diff: add, replacing cmd/internal/diff
This is an in-process (non-exec'ing) replacement for cmd/internal/diff.
It uses an O(n log n) algorithm instead of the O(n²) algorithm
in standard diff binaries. It does not produce the absolute
shortest diffs, but the results are often more meaningful
than the standard diff, because it doesn't try to align
random blank lines or other noise.
Adding so that tests inside std (especially go/printer)
can print diffs.
Replacing cmd/internal/diff because we don't need two.
Change-Id: I9155dd925e4a813f5bfa84a8ad3dec8ffdbf8550
Reviewed-on: https://go-review.googlesource.com/c/go/+/384255
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>
Trust: Peter Weinberger <pjw@google.com>