instructions to update pull request makes assumptions about git remotes (integration)

[boneskull]

In mochajs/mocha#2621, the automated message contains the following:

There are at least two problems here:

1. My working copy's origin is not github.com/mochajs/mocha, it's github.com/boneskull/mocha.

   I believe you can get away with just giving git the URL:

   $ git push ssh://git@github.com/mochajs/mocha.git greenkeeper/update-all

   I don't know if it's better to use an https scheme.

2. git fetch assumes origin. Again, a specific URL, or git fetch --all may be better.

I realize it isn't really Greenkeeper's job to teach developers how to use git. Either way, had I followed these instructions, it wouldn't work.