This patch adds the standard issue option (-i/--issue) to
"git cl try" command.

This is useful for maintaining FlagExpectations with fyi bots.
Runtime-flag features often see new failures, and a good number
of tests are flaky. Maintainers can upload a CL and use cron to
run try jobs to collect multiple tests results to find new
failures and flaky tests.

Change-Id: I069593950023ddae814faf3074c33de4c98cb70e
Reviewed-on: https://chromium-review.googlesource.com/850792Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>