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 needed to support a new flag (`IncludeLfsBlobs`) in the
`GetArchiveRequest` RPC.

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
```

changelogs/unreleased/sh-update-gitaly-dep.yml

+---
+title: Update Gitaly module dependency
+merge_request: 619
+author:
+type: added

go.mod

@@ -24,7 +24,7 @@ require (
 	github.com/rafaeljusto/redigomock v0.0.0-20190202135759-257e089e14a1
 	github.com/sebest/xff v0.0.0-20160910043805-6c115e0ffa35
 	github.com/shabbyrobe/gocovmerge v0.0.0-20190829150210-3e036491d500 // indirect
-	github.com/sirupsen/logrus v1.3.0
+	github.com/sirupsen/logrus v1.6.0
 	github.com/smartystreets/goconvey v1.6.4
 	github.com/stretchr/testify v1.5.1
 	gitlab.com/gitlab-org/gitaly v1.74.0
@@ -37,3 +37,8 @@ require (
 	gopkg.in/yaml.v2 v2.2.8 // indirect
 	honnef.co/go/tools v0.0.1-2020.1.5
 )
+
+// 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 v1.87.1-0.20201001041716-3f5e218def93