update.sh: remove stale git-svn *.lock files before git svn fetch
Some git svn fetch updates take an abnormally long time even when
retrieving just a single new revision.
If the git svn fetch gets killed because of a jobd.pl timeout,
then it may leave a stale *.lock file behind which will prevent
the next git svn fetch from doing anything.
This change removes stale lock files at least 60 minutes old.
This way there will be a pause before resuming a dreadfully
long git svn fetch that was killed due to a timeout as the
stale *.lock files must age to at least 1 hour first.