PSA: gclient sync with git cache will no longer unnecessarily fetch newest commits of each dependency

[Andrii Shyshkalov]

tl;dr if your tooling relies on gclient sync to fetch not-yet-rolled commits of DEPS, this will stop working. If you don't use git cache backed gclient checkout (most developers), you won't be affected.

This CL changes gclient sync behavior when fetching dependencies backed by git cache for which exact pinned revision is known. Previously, git cache would be updated unconditionally. Once my CL lands, git cache will be updated only if desired revision isn't yet there.

So, if your recipe/tool is affected by this, you should explicitly run "git cache populate" on the cache dir you need.

If you have any concerns, please let me know here or by commenting on CL.

Thanks,

Andrii