Wiki links broken on self-hosted Gitlab (Path to relative URL is doubled)

Summary

The /gitlab path to Gitlab is rendered as /gitlab/gitlab in Markdown links.

Steps to reproduce

# gitlab.rb
external_url "https://DOMAIN/gitlab"

The wiki page https://DOMAIN/gitlab/mygroup/myproj/-/wikis/some_page:

[MyLinkText](my-link-slug)

What is the current bug behavior?

On page some_page, the link MyLinkText links to https://DOMAIN/gitlab/gitlab/mygroup/myproj/-/wikis/my-link-slug

What is the expected correct behavior?

On page some_page, the link MyLinkText should link to https://DOMAIN/gitlab/mygroup/myproj/-/wikis/my-link-slug

Results of GitLab environment info

Expand for output related to GitLab environment info 
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

root@mfix:/etc/gitlab# gitlab-rake gitlab:env:info

System information
System:
Current User:	git
Using RVM:	no
Ruby Version:	2.6.6p146
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.9
Git Version:	2.26.2
Sidekiq Version:5.2.7
Go Version:	unknown

GitLab information
Version:	13.0.6
Revision:	5aa982e01ea
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	11.7
URL:		https://mfix.netl.doe.gov/gitlab
HTTP Clone URL:	https://mfix.netl.doe.gov/gitlab/some-group/some-project.git
SSH Clone URL:	git@mfix.netl.doe.gov:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:

GitLab Shell
Version:	13.2.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check 
(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
root@mfix:/etc/gitlab# gitlab-rake gitlab:check SANITIZE=true
Checking GitLab subtasks ...


Checking GitLab Shell ...


GitLab Shell: ... GitLab Shell version >= 13.2.0 ? ... OK (13.2.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful


Checking GitLab Shell ... Finished


Checking Gitaly ...


Gitaly: ... default ... OK


Checking Gitaly ... Finished


Checking Sidekiq ...


Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1


Checking Sidekiq ... Finished


Checking Incoming Email ...


Incoming Email: ... Reply by email is disabled in config/gitlab.yml


Checking Incoming Email ... Finished


Checking LDAP ...


LDAP: ... LDAP is disabled in config/gitlab.yml


Checking LDAP ... Finished


Checking GitLab App ...


Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
4/7 ... yes
12/8 ... yes
22/11 ... yes
26/12 ... yes
22/13 ... yes
25/16 ... yes
26/18 ... yes
25/20 ... yes
14/21 ... yes
33/24 ... yes
32/27 ... yes
11/28 ... yes
25/29 ... yes
10/30 ... yes
37/32 ... yes
12/33 ... yes
33/34 ... yes
22/35 ... yes
18/37 ... yes
6/38 ... yes
25/39 ... yes
25/41 ... yes
33/42 ... yes
33/44 ... yes
26/45 ... yes
4/46 ... yes
36/47 ... yes
33/48 ... yes
22/49 ... yes
33/50 ... yes
4/52 ... yes
33/54 ... yes
25/56 ... yes
53/57 ... yes
54/58 ... yes
25/59 ... yes
6/60 ... yes
47/61 ... yes
25/62 ... yes
26/64 ... yes
4/65 ... yes
4/67 ... yes
10/68 ... yes
41/71 ... yes
74/73 ... yes
73/74 ... yes
4/76 ... yes
73/77 ... yes
71/81 ... yes
71/83 ... yes
4/84 ... yes
73/85 ... yes
22/86 ... yes
73/87 ... yes
90/89 ... yes
73/94 ... yes
73/95 ... yes
74/96 ... yes
74/97 ... yes
73/98 ... yes
97/99 ... yes
98/100 ... yes
90/101 ... yes
86/102 ... yes
74/103 ... yes
97/105 ... yes
73/107 ... yes
92/108 ... yes
102/109 ... yes
4/110 ... yes
26/113 ... yes
97/114 ... yes
9/116 ... yes
99/117 ... yes
90/118 ... yes
102/119 ... yes
8/123 ... yes
22/124 ... yes
97/125 ... yes
22/126 ... yes
22/127 ... yes
73/128 ... yes
97/130 ... yes
90/131 ... yes
102/133 ... yes
25/135 ... yes
62/136 ... yes
26/140 ... yes
118/141 ... yes
102/142 ... yes
25/143 ... yes
94/145 ... yes
4/146 ... yes
131/147 ... yes
131/148 ... yes
90/149 ... yes
90/150 ... yes
73/151 ... yes
71/152 ... yes
5/153 ... yes
74/154 ... yes
5/155 ... yes
103/156 ... yes
4/157 ... yes
103/158 ... yes
97/159 ... yes
97/160 ... yes
9/161 ... yes
103/162 ... yes
131/163 ... yes
130/164 ... yes
130/165 ... yes
102/166 ... yes
102/167 ... yes
22/169 ... yes
97/170 ... yes
4/171 ... yes
22/172 ... yes
128/173 ... yes
156/174 ... yes
157/177 ... yes
103/179 ... yes
160/180 ... yes
25/181 ... yes
158/182 ... yes
90/183 ... yes
160/184 ... yes
10/185 ... yes
5/186 ... yes
165/187 ... yes
4/188 ... yes
74/189 ... yes
169/191 ... yes
103/192 ... yes
170/193 ... yes
103/194 ... yes
74/195 ... yes
5/196 ... yes
10/197 ... yes
103/198 ... yes
34/199 ... yes
34/200 ... yes
34/201 ... yes
10/202 ... yes
10/203 ... yes
9/204 ... yes
4/205 ... yes
103/206 ... yes
10/207 ... yes
181/210 ... yes
78/211 ... yes
10/212 ... yes
103/213 ... yes
5/216 ... yes
5/218 ... yes
3/219 ... yes
103/220 ... yes
10/221 ... yes
105/222 ... yes
72/223 ... yes
10/224 ... yes
177/225 ... yes
131/226 ... yes
8/227 ... yes
103/228 ... yes
102/230 ... yes
102/231 ... yes
102/232 ... yes
194/234 ... yes
194/235 ... yes
188/236 ... yes
4/237 ... yes
5/238 ... yes
167/239 ... yes
109/240 ... yes
111/243 ... yes
127/244 ... yes
196/245 ... yes
194/246 ... yes
79/247 ... yes
90/248 ... yes
199/249 ... yes
90/250 ... yes
188/251 ... yes
10/252 ... yes
10/253 ... yes
162/254 ... yes
206/255 ... yes
5/258 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.6)
Git version >= 2.22.0 ? ... yes (2.26.2)
Git user has default SSH configuration? ... yes
Active users: ... 170
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished