[auth] Close stdin for git-credential-luci

Per the Git credential helper protocol, the credential helpers should
read stdin for attributes.  Previously, we didn't implement the
attributes for git-credential-luci, so this wasn't an issue, but
we're (going to) use the attributes now, and if we don't explicitly
send or close stdin, the helper will hang

Bug: 411487838
Change-Id: I80d47c31e597187c05d5fc23438ae8a52147cb9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6868130
Commit-Queue: Gavin Mak <gavinmak@google.com>
Auto-Submit: Allen Li <ayatane@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

auth.py

@@ -240,6 +240,7 @@ class GerritAuthenticator(object):
         try:
             out, err = subprocess2.check_call_out(
                 ['git-credential-luci', 'get'],
+                stdin=subprocess2.DEVNULL,
                 stdout=subprocess2.PIPE,
                 stderr=subprocess2.PIPE)
             logging.debug('git-credential-luci stderr:\n%s', err)