git-review: expand pending output

for pending:
- show full commit message
- show information about being behind upstream
- show list of modified files
- for current branch, show staged, unstaged, untracked files
- warn about being ahead of upstream on master
- warn about being multiple commits ahead of upstream

- add same warnings to change
- add change -a (mostly unrelated, but prompted by this work)

Change-Id: Ie480ba5b66cc07faffca421ee6c9623d35204696
Reviewed-on: https://go-review.googlesource.com/1620
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>