cmd/gitlock: new tool to lock git dependencies in place in Dockerfiles

Fixes golang/go#20747

Change-Id: I8fe03af0fb8db4d1fd7c396d7b228b87209a9208
Reviewed-on: https://go-review.googlesource.com/47971
Reviewed-by: Kevin Burke <kev@inburke.com>
14 files changed