bot_update: don't retry rebasing Gerrit patches.

It's pointless, and sleeping between retries adds unnecessary delays
which wastes capacity and developers time.

BUG=690940
R=hinoka@chromium.org

Change-Id: I80cf52f04ed74e2407c373326a8c6f52a3a7d958
Reviewed-on: https://chromium-review.googlesource.com/458439
Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

recipe_modules/bot_update/resources/bot_update.py

@@ -679,7 +679,7 @@ def apply_gerrit_ref(gerrit_repo, gerrit_ref, root, gerrit_reset,
         ok = False
         git('checkout', '-b', temp_branch_name, cwd=root)
         try:
-          git('rebase', base_rev, cwd=root)
+          git('rebase', base_rev, cwd=root, tries=1)
         except SubprocessFailed:
           # Abort the rebase since there were failures.
           git('rebase', '--abort', cwd=root)