License key throws Error 500 when HEAD ponts to a ref that does not exist

If HEAD is refs/heads/master but the branch master does not exist, an Error 500 occurs:

https://sentry.gitlap.com/gitlab/gitlabcom/issues/4621/

Rugged::ReferenceError: Reference 'refs/heads/master' not found
  lib/gitlab/metrics/instrumentation.rb:153:in `head'
    retval   = super
  lib/gitlab/metrics/instrumentation.rb:153:in `head'
    retval   = super
  lib/gitlab/metrics/instrumentation.rb:153:in `last_commit'
    retval   = super
  app/models/repository.rb:545:in `block in license_key'
    Licensee.license(path).try(:key)
  lib/repository_cache.rb:19:in `fetch'
    backend.fetch(cache_key(key), &block)
...
(150 additional frame(s) were not displayed)

ActionView::Template::Error: Reference 'refs/heads/master' not found
  lib/gitlab/metrics/instrumentation.rb:153:in `head'
    retval   = super
  lib/gitlab/metrics/instrumentation.rb:153:in `head'
    retval   = super
  lib/gitlab/metrics/instrumentation.rb:153:in `last_commit'
    retval   = super
  app/models/repository.rb:545:in `block in license_key'
    Licensee.license(path).try(:key)
  lib/repository_cache.rb:19:in `fetch'
    backend.fetch(cache_key(key), &block)
...
(150 additional frame(s) were not displayed)

mentioned in merge request !4151 (merged)

mentioned in issue gitlab-com/support-forum#710 (closed)

mentioned in issue #17519 (closed)

Reassigned to @stanhu

Status changed to closed by merge request !4151 (merged)

mentioned in commit dec6b31c

mentioned in commit 56eb4200

mentioned in commit f2e46514

mentioned in issue #17590 (closed)

mentioned in issue #21038 (closed)

mentioned in merge request !14785