`git-rv export` should not populate metadata if no review is in progress and no changes are available to be uploaded

[dhermes]

Currently, git rv export will run even if there are no commits, because it could potentially update the metadata, i.e. a person may want to change the issue subject without sending any changes:

git rv export --message="I didn't do anything. Ha!"

However, this still works before a review has begun, which should not occur.

Example:

• First clone a new repo

$ git clone https://github.com/GoogleCloudPlatform/git-rv
$ cd git-rv

• Do absolutely nothing
• Try to export your nothing changes

$ git rv export -r reviewer@mail.com
No output from ['git', 'diff', '--no-color', '--no-ext-diff', '--full-index', '--ignore-submodules', SOMEHASH']

and realize that upload.py refused to upload anything, this ending the command.

• Check to make sure nothing bad happened

$ git rv getinfo
{
  "reviewers": [
    "reviewer@mail.com"
  ], 
  "private": false, 
  "remote_info": {
    "url": "https://github.com/GoogleCloudPlatform/git-rv", 
    "last_synced": "SOMEHASH", 
    "remote": "origin", 
    "commit_hash": "SOMEHASH", 
    "branch": "master"
  }, 
  "server": "codereview.appspot.com"
}

Oh no!

/cc @fredsa