We've noticed that if zuul executors (and presumably mergers) don't shut
down gracefully that they may leak git index.lock files in the .git dirs
of the merger repos. Since these repos should be dedicated to zuul's use
without outside interference we can reasonably safely remove any present
index.lock files when starting zuul mergers (and executors).
This implementation does an os.walk under the merger repos root looking
for .git dirs and once it has found them checks for any index.lock
files. This happens before starting the gearman worker which should
avoid any races with these resources.