git-cache: pack refs to reduce # of files downloaded.

I've observed today that V8 builder spends ~1 minute to download
a bunch of lose files representing tags[1]. This CL should
shrink ~2K files to just 72.

[1]
https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8865281064556718960/+/steps/initialization/0/steps/bot_update/0/stdout

R=jochen@chromium.org, machenbach@chromium.org

Change-Id: Ib8f245e6ed67ea91b53b22a8e4f729ab899a1cbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2502476
Auto-Submit: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cache.py

@@ -495,6 +495,9 @@ class Mirror(object):
       print('Cache %s already exists.' % dest_prefix)
       return
 
+    # Reduce the number of individual files to download & write on disk.
+    self.RunGit(['pack-refs', '--all'])
+
     # Run Garbage Collect to compress packfile.
     gc_args = ['gc', '--prune=all']
     if gc_aggressive: