Shorten module names from Snaps_worker_* to Snaps_work*
Add exception-full interface to git add/commit.
1st stab at progress stats updates.

Ref #19
Remove redundant call to shutdown.

Async.Command already does it for us:
https://github.com/janestreet/async_extra/commit/5ef9d24f8a41595b422fbc2933ea394d1459f6c4
https://github.com/janestreet/async_extra/blob/5ef9d24f8a41595b422fbc2933ea394d1459f6c4/lib/command.ml
Rename channels to more informative names.
Re-align and re-format.
Retry git operation after attempted cleanup.

Ref #18
Fix logging interface. "init" is a global call.
Log to file instead of stderr.

Ref #19
Reset cursor position for cleaner progress output.
Add missing whitespace.