review: overhaul of tool user interface

The 'create' and 'commit' commands are combined to become 'change',
which also handles switching branches.

Also, move the various commands to their own source files.

Change-Id: Ia9a1a5d27bdebdb2fad057e9069855c935cbcb21
Reviewed-on: https://go-review.googlesource.com/1100
Reviewed-by: Russ Cox <rsc@golang.org>