Fixes #634: Clears git locks when the deployment lock is acquired.

Fixes #634: Clears git locks when the deployment lock is acquired.

Added ClearLock() when DeploymentLock is acquired to prevent deadlock in more scenarios.

For example, the scenario below will return an error. 
1. Run Kudu. Assume $KUDU_DIR$ is .../kudu/apps/<app name>/site/repository
2. To simulate git crashing during an operation, create the index.lock file in .git folder. In msys git, run 
    touch $KUDU_DIR$/.git/index.lock
3. Try the api in LiveScmEditorController, which will fail.
    curl -X PUT --data " cool page" %KUDU_URL%/scmvfs/cool.txt
An error will be returned.

The idea is that index.lock may be left over from a previous failed git operation, which can be cleared for users.