Added GIT_SYNC_ONE_TIME environment variable

[cmerris-inmoment]

When true, fetch only happens once and program terminates. When false, fetch happens continually, as before, using the GIT_SYNC_WAIT environment variable as a polling interval.

*For my team's usage, this was the only piece missing from git-sync, i.e. the ability to just pull once without polling. We are going to be using git-sync as part of a microservice orchestration tool which builds and runs repos. Thanks for porting this over from k8s!