Remove branch option from git cache update-bootstrap.

in update_bootstrap, it's only used for `git number`.
without it git number defaults to `HEAD`
and
nothing seems to actually set the `branch` option
https://source.chromium.org/search?q=%22update-bootstrap%22

Should fix the following failure: http://shortn/_nLLz2nJAGS

Change-Id: I5f3eb8497133c1e5d2f964af969ca61ab6256330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4080542
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>

git_cache.py

@@ -509,7 +509,7 @@ class Mirror(object):
         self._fetch(self.mirror_path, verbose, depth, no_fetch_tags,
                     reset_fetch_config)
 
-  def update_bootstrap(self, prune=False, gc_aggressive=False, branch='main'):
+  def update_bootstrap(self, prune=False, gc_aggressive=False):
     # NOTE: There have been cases where repos were being recursively uploaded
     # to google storage.
     # E.g. `<host_url>-<repo>/<gen_number>/<host_url>-<repo>/` in GS and
@@ -524,9 +524,9 @@ class Mirror(object):
       gclient_utils.rmtree(recursed_dir)
 
     # The folder is <git number>
-    gen_number = subprocess.check_output(
+    gen_number = subprocess.check_output([self.git_exe, 'number'],
-        [self.git_exe, 'number', branch],
+                                         cwd=self.mirror_path).decode(
-        cwd=self.mirror_path).decode('utf-8', 'ignore').strip()
+                                             'utf-8', 'ignore').strip()
     gsutil = Gsutil(path=self.gsutil_exe, boto_path=None)
 
     dest_prefix = '%s/%s' % (self._gs_path, gen_number)
@@ -655,8 +655,6 @@ def CMDupdate_bootstrap(parser, args):
                     help='Run aggressive repacking of the repo.')
   parser.add_option('--prune', action='store_true',
                     help='Prune all other cached bundles of the same repo.')
-  parser.add_option('--branch', default='main',
-                    help='Branch to use for bootstrap. (Default \'main\')')
 
   populate_args = args[:]
   options, args = parser.parse_args(args)
@@ -671,7 +669,7 @@ def CMDupdate_bootstrap(parser, args):
   _, args2 = parser.parse_args(args)
   url = args2[0]
   mirror = Mirror(url)
-  mirror.update_bootstrap(options.prune, options.gc_aggressive, options.branch)
+  mirror.update_bootstrap(options.prune, options.gc_aggressive)
   return 0