git cl creds-check: understand '~' in path to .gitcookies.

Before, git cl creds-check expected `http.cookiefile` of git config
to have path without '~' which git understands on Linux/Mac.

R=machenbach@chromium.org

Bug: 719728
Change-Id: Ica1308b776b62bda7a8b85113c0c0df837c7d933
Reviewed-on: https://chromium-review.googlesource.com/501828Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cl.py

@@ -3648,6 +3648,7 @@ class _GitCookiesChecker(object):
     default = gerrit_util.CookiesAuthenticator.get_gitcookies_path()
     configured_path = RunGitSilent(
         ['config', '--global', 'http.cookiefile']).strip()
+    configured_path = os.path.expanduser(configured_path)
     if configured_path:
       self._ensure_default_gitcookies_path(configured_path, default)
     else: