abd: run in a loop and sleep between fetches
abd: run in a loop and sleep between fetches

In order to make abd usable on the command-line, provide a new option
'--sleep-duration-secs' which specifies how long abd should sleep
between fetches of the current repo.  Abd will now run in an infinite
loop, until quit with Ctrl-C.

Test Plan: ran on ph-tools repo with various delays
Reviewers: jevripio
Differential Revision: https://sibld.phab.dev.bloomberg.com/D110

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