if the sanity bot is configured with gitdofetch=1, the repos need a
periodic cleanup - otherwise they become amazingly slow (a simple fetch
can take minutes before anything happens).

i was too lazy make this an option of the sanity-bot script itself.
maybe later.

Change-Id: I85306264cd67d321c10cb069e5bfb5b60b2ca6db
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>