Increased incidents of Error 500s due to inconsistent branch cache state

https://sentry.gitlap.com/gitlab/gitlabcom/issues/8756/

TypeError: wrong argument type nil (expected String)
  lib/gitlab/metrics/instrumentation.rb:152:in `block in rev_parse'
    trans.measure_method(#{label.inspect}) { super }
  lib/gitlab/metrics/method_call.rb:23:in `measure'
    retval = yield
  lib/gitlab/metrics/transaction.rb:71:in `measure_method'
    @methods[name].measure(&block)
  lib/gitlab/metrics/instrumentation.rb:152:in `rev_parse'
    trans.measure_method(#{label.inspect}) { super }
  lib/gitlab/metrics/instrumentation.rb:152:in `block in rev_parse_target'
    trans.measure_method(#{label.inspect}) { super }
...
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