bot_update: attempt to disable GC completely on main repo.

Motivation: see https://crbug.com/822437#c6
tl;dr apparently, auotpacking still happens, this CL tries to stop
prevent it harder than before.

R=vadimsh@chromium.org

Bug: 822437
Change-Id: Idda384bdebd96c10ac79c48b0622f84550060362
Reviewed-on: https://chromium-review.googlesource.com/1045491Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -752,6 +752,7 @@ def _git_checkout(sln, sln_dir, revisions, shallow, refs, git_cache_dir,
         raise
 
 def _git_disable_gc(cwd):
+  git('config', 'gc.auto', '0', cwd=cwd)
   git('config', 'gc.autodetach', '0', cwd=cwd)
   git('config', 'gc.autopacklimit', '0', cwd=cwd)