git_cache: allow self-bootstrapping when updating bootstrap.

The only upside of fetching everything directly from GoB
is that git cache would get just 1 .pack. However, git cache
already runs "git gc" before uploading cache to GS, so
this point is moot.

R=maruel

Bug: 943696
Change-Id: Ie8e77a81aa81489dae240b7c767c5842a12c6f19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1585641
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cache.py

@@ -720,11 +720,10 @@ def CMDupdate_bootstrap(parser, args):
     return 1
 
   parser.add_option('--prune', action='store_true',
-                    help='Prune all other cached zipballs of the same repo.')
+                    help='Prune all other cached bundles of the same repo.')
 
   # First, we need to ensure the cache is populated.
   populate_args = args[:]
-  populate_args.append('--no-bootstrap')
   CMDpopulate(parser, populate_args)
 
   # Get the repo directory.