Remove stale HEAD.lock if it exists

Zeger-Jan van de Wegrequested to merge
zj-head-lokc into master

Stale HEAD.lock files can make merging impossible for users, as reported in https://gitlab.com/gitlab-org/gitlab-ce/issues/50042

This changes removes HEAD.lock files older than 15 minutes, like we do with config.lock.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50042

Edited by GitLab Release Tools Bot

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