git-cache: don't limit pack files to 2GiB.

This is only relevant to 1 builder which updates git cache bundles.
Since some time in 2018-2019, it started creating several 2GiB packs
for chromium/src. I verified that the setting affecting it is
  core.deltaBaseCacheLimit

Log file of a build which I observed while ssd-ed into the machine (internal):
  https://logs.chromium.org/logs/infra-internal/buildbucket/cr-buildbucket.appspot.com/8901682638839779824/+/steps/Updating_https:__chromium.googlesource.com_chromium_src/0/stdout
Git version: version:2.21.0.chromium16

R=ehmaldonado

Change-Id: I52cadfb9f34faea09a57d53387cab7e0538362b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1818076
Auto-Submit: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cache.py

@@ -592,6 +592,13 @@ class Mirror(object):
       return
 
     # Run Garbage Collect to compress packfile.
+    # It appears core.deltaBaseCacheLimit restricts the file size of the pack
+    # files produced, while we want 1 big pack file for efficiency.
+    try:
+      self.RunGit(['config', '--unset-all', 'core.deltaBaseCacheLimit'],
+                  cwd=cwd)
+    except subprocess.CalledProcessError:
+      pass  # Nothing to unset is fine.
     gc_args = ['gc', '--prune=all']
     if gc_aggressive:
       gc_args.append('--aggressive')