Use glg/deploycache as an option for checking for GIT updates

[datfinesoul]

Overview

• /overlay/etc/starphleet contains new GLG_DEPLOYCACHE_URL env var, which turns on feature if set.
• /scripts/starphleet-git-sync contains new logic to only fetch updates from github if the new glg/deploycache service shows a difference in SHA from what's currently in the local repos on starphleet.

• The idea is, instead of performing a fetch against github to see if a new commit is available, we check a cached version of the SHA, per org+repo+branch. If the cache shows a change, we execute the standard starphleet workflow.
• Outside of the SHA difference detection, there are no workflow changes to starphleet

Related to https://github.com/glg/metadevops-issues/issues/213

Sample stats from build server

The line in bold/italic is what we are addressing which is causing GitHub to block us.

Stats: starting with 53337 events

• No Repo "not found, initial clone needed from"
  • clone 614
• Have Repo
  • Different Remote "repository url differs, reclone needed"
  • clone 1
  • Same Remote
  • fetch 52723 (FIXING THIS)
  • Different Branch "specified branch changed, checking out"
    • clone 3
  • Out of Date "new code detected, pulling"
    • fetch 122
    • reset
  • Not current "this clone is not current, reclone needed"
    • clone 0
  • Divergent "this clone is diverged, reclone needed"
    • clone 0

fatal error 0 no update 52598

[datfinesoul]

closing in favor of glg/starphleet#2