Go get fails with the usage of subgroups

Steps to reproduce

Go get seems not able to fetch the correct path when using subgroups:

$ go get -v xxx.local/a/b/project
Fetching https://xxx.local/a/b/project?go-get=1
Parsing meta tags from https://xxx.local/a/b/project?go-get=1 (status code 200)
get "xxx.local/a/b/project": found meta tag main.metaImport{Prefix:"xxx.local/a/b", VCS:"git", RepoRoot:"https://xxx.local/a/b.git"} at https://xxx.local/a/b/project?go-get=1
get "xxx.local/a/b/project": verifying non-authoritative meta tag
Fetching https://xxx.local/a/b?go-get=1
Parsing meta tags from https://xxx.local/a/b?go-get=1 (status code 200)
xxx.local/a/b (download)
package xxx.local/a/b/project: /home/user/go/src/xxx.local/a/b exists but /home/user/go/src/xxx.local/a/b/.git does not - stale checkout?

The issue here is that the RepoRoot is wrong, because it points to https://xxx.local/a/b.git instead of https://xxx.local/a/b/project.git. What works actually without any error is:

$ go get -v xxx.local/a/b/project.git

What is the current bug behavior?

The repo is not go get-able.

What is the expected correct behavior?

It should work with and without the .git extension.

Results of GitLab environment info

System information
System:         Ubuntu 16.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.3.3p222
Gem Version:    2.6.6
Bundler Version:1.13.7
Rake Version:   10.5.0
Redis Version:  3.2.5
Git Version:    2.10.2
Sidekiq Version:4.2.7

GitLab information
Version:        9.0.5-ee
Revision:       71d72c9
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
DB Version:     9.6.1
URL:            https://xxx.local
HTTP Clone URL: https://xxx.local/some-group/some-project.git
SSH Clone URL:  git@xxx.local:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     yes
Using Omniauth: no

GitLab Shell
Version:        5.0.0
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:            /opt/gitlab/embedded/bin/git