Arcyd: fix arcyd status file unlimited growth

Stop the Arcyd status file from growing with each cycle - update
existing entries for repositories instead of continuously appending to
the list.

Test Plan:
Add 'repeated repo' to dev-status-html
Manually inspect $ ./testbed/arcyd/status_html_testbed.sh
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh