cmd/gorename: -d flag prints diffs instead of rewriting files

The user may specify the diff tool using the -diffcmd flag.

+ test.

Also:
- eliminate redundant DryRun flag
- simplify Verbose messages using log.SetPrefix

Fixes issue #13355

Change-Id: I917edc73e31ddf0f5d5b9b30c43f826465529da1
Reviewed-on: https://go-review.googlesource.com/18208
Reviewed-by: Michael Matloob <matloob@golang.org>
6 files changed