Update Gitaly module dependency

This updates Gitaly's latest Protocol Buffer definitions, which were
last updated in Feburary 2020, with the latest version of Gitaly. This
is done to ensure the definitions are in sync with Workhorse
(https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/619)
and Gitaly.

Because we started tagging Gitaly versions `v13.x.x` instead of
`v1.x.x`, `go get` refuses to update the Gitaly module using the latest
tag or SHA (https://gitlab.com/gitlab-org/gitaly/-/issues/3177).  Go
tries to enforce semantic compatibility by requiring the use of module
paths (e.g. `m/v13`), and Gitaly doesn't adhere to this scheme.

To workaround this issue, we use the replace directive
(https://github.com/golang/go/wiki/Modules#when-should-i-use-the-replace-directive):

```
go mod edit -replace gitlab.com/gitlab-org/gitaly=gitlab.com/gitlab-org/gitaly@3f5e218def93024f3aafe590c22cd1b29f744105
```

go.mod

@@ -5,11 +5,15 @@ go 1.13
 require (
 	github.com/mattn/go-shellwords v0.0.0-20190425161501-2444a32a19f4
 	github.com/otiai10/copy v1.0.1
-	github.com/otiai10/curr v1.0.0 // indirect
-	github.com/sirupsen/logrus v1.3.0
+	github.com/sirupsen/logrus v1.6.0
 	github.com/stretchr/testify v1.4.0
 	gitlab.com/gitlab-org/gitaly v1.68.0
-	gitlab.com/gitlab-org/labkit v0.0.0-20200507062444-0149780c759d
+	gitlab.com/gitlab-org/labkit v0.0.0-20200908084045-45895e129029
 	google.golang.org/grpc v1.24.0
-	gopkg.in/yaml.v2 v2.2.4
+	gopkg.in/yaml.v2 v2.2.8
 )
+
+// go get tries to enforce semantic version compatibility via module paths.
+// We can't upgrade to Gitaly v13.x.x from v1.x.x without using a manual override.
+// See https://gitlab.com/gitlab-org/gitaly/-/issues/3177 for more details.
+replace gitlab.com/gitlab-org/gitaly => gitlab.com/gitlab-org/gitaly v0.0.0-20201001041716-3f5e218def93