cmd/fix: always format source file before fixing

This makes the changes to the file easier to explain.
Not all the changes may come from the fixers directly,
if the file is not gofmt-ed already.

Change-Id: I81776da446a34a1239a3130317d2aae1437d61a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/240555


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>