Monitor 99-th percentile for FilterShasWithSignatures (go implementation)

After moving to the go implementation of FilterShasWithSignatures method (protected with gitaly_filter_shas_with_signatures_go feature flag) the Median Latency shows better results, but the 99th Percentile Latency changed drammatically, please checkout on the dashboard. Maybe it is worth to change call to log.GetCommitCatfile(c, string(sha)) to something more simpler to omit additional git operation c.Info(revision + "^{commit}").