Enhancement: Allow different urls for push and fetch

[GoogleCodeExporter]

Affected Version: 1.21

We are using repo to fetch different repos but without gerrit.
The repositories are fetched via HTTPS in read only mode. Developers with push 
access will push to the repositories via SSH.

So for us it would be nice to be able to specify an additional URL for each 
remote.

Proposal:
Currently a remote is defined like this:
<remote fetch="https://gitserver.local/mirror/" name="mirror" />

With the extension it would be possible to write:
<remote fetch="https://gitserver.local/mirror/" 
push="git@gitserver.local/mirror.git" />

Original issue reported on code.google.com by pasci.bach@gmail.com on 7 Jan 2015 at 9:28