On pull git-backup locks backup repository to make sure another
concurrent `git-backup pull` process is not running. However until now,
if a pull was failing, the lock was left unreleased, which made followup
pull attempts to fail while acquiring the lock until the lock was
manually removed with `git update-ref -d ...`. Probably originally I
made it like this in 6f237f22 (git-backup: Initial draft) to make sure
that if there is a problem it does not go unnoticed and forces me to
investigate. But in general we do _not_ need to keep the lock on error
return after `git-backup pull` completes even abnormally.

This "lock left unreleased" is causing operational issues on
lab.nexedi.com from time to time: if a pull try fails for some, even
temporary, reason, all next pull tries will fail until a human intervene
and remove the lock ref.

Fix it.

See also: !4