Add origin remote if it's not already set on the mirror repo

[KevinGreen]

Problem

We've hit an issue a couple times where agents with a shared git mirror repo fail when trying to set the remote url for origin. The agents sharing the repo will fail consistently but can be recovered by manually running git --git-dir <mirrorDir> remote add origin <url> on the host.

The root cause for the repo getting into this state isn't known yet. From scanning the logs I haven't found any errors or abnormal operation. The only thing I see is that all the agents start picking up jobs at the same time so it could be due to another git operation running without a lock somewhere else.

Proposed Solution

This PR just checks if there's not already a url for origin and runs git remote add if there isn't. This code runs within the update lock so there won't be any issue with multiple agents checking and branching based on the git remote get-url result.

Tests

There aren't any unit tests for this part of the code as far as I've seen. This doesn't reproduce often but we want to have a fix in to avoid issues in the future.