git-codereview: make change fail with multiple pending changes

When using 'git change' to amend the current change, it will fail
if there are multiple pending changes. The previous behavior (amending
the latest) wasn't always what the user intended.

    $ git change
    git-codereview: cannot amend change: multiple changes pending:
    	fc60fd5 test: dummy change
    	6440cdc git-codereview: make change fail with multiple pending changes
    $

Unlike mail and submit, change doesn't support explicitly telling
which commit to work on. Either 'git commit --amend' (for updating
the topmost change) or a combination of 'git commit' and 'git rebase'
must be used.

Fixes golang/go#10443.

Change-Id: I2f6ea1b0e302b66b627761d72d031a3b6aa0fba2
Reviewed-on: https://go-review.googlesource.com/20049
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
5 files changed