detect deployed git HEAD in azure

When deployed on MS azure, the available git commit used to deploy the
software is available in the SCM_COMMIT_ID environment variable.
We encourage all hosting providers to provide some similar facility to
avoid spawning spurious processes.

Thanks to @shanselman for reminders and pointers to the fix.
Merge pull request #213 from nightscout/wip/azure/git-head

detect deployed git HEAD in azure