git-review: simplify sync
git-review: simplify sync

The old sync was needlessly complex. It separated fetch and rebase,
and it had a special case to do a fast-forward merge if possible.
Git pull -r will do all that in one step, and using one command
instead of three simplifies the 'git sync -v' output for people who
want to know what git sync really does.

This also matches the documentation we published in Go 1.4
at golang.org/doc/contribute.html.

Change-Id: Ic367d0300c30120a900c6b4d66546d2e0a8fb6ce
Reviewed-on: https://go-review.googlesource.com/1426
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

Go-zh/review,golang/review