review: move 'review' command to 'git-review' sub-directory

If 'git-review' is in your path, 'git review' will invoke 'git-review'.

Change-Id: I0f1134776dbbf4bd5c7619248b69d1b5ba884162
Reviewed-on: https://go-review.googlesource.com/1160
Reviewed-by: Rob Pike <r@golang.org>