repo: point default branch to repo-1

Since this will be feature-frozen for Python 2 users, lets point the
default update branch to "repo-1" rather than "stable" as the latter
will follow the master development (and Python 3-only).

Bug: https://crbug.com/gerrit/10418
Change-Id: Iceff0983684a580dc5c9ec1c60acfb5eda5ce2c4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253172
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

repo

@@ -16,7 +16,7 @@ import os
 REPO_URL = os.environ.get('REPO_URL', None)
 if not REPO_URL:
   REPO_URL = 'https://gerrit.googlesource.com/git-repo'
-REPO_REV = 'stable'
+REPO_REV = 'repo-1'
 
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