cmd/gofmt: use actual filename in gofmt -d output

cmd/gofmt: use actual filename in gofmt -d output

By using actual filename, diff output of "gofmt -d" can be used with
other commands like "diffstat" and "patch".

Example:
  $ gofmt -d path/to/file.go | diffstat
  $ gofmt -d path/to/file.go > gofmt.patch
  $ patch -u -p0 < gofmt.patch

Fixes #18932

Change-Id: I21ce15eb77870d72f2c14bfd5e7c21e2c77dc9ab
Reviewed-on: https://go-review.googlesource.com/36374
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>