Relative URLs in markdown documents are incorrect

Created by: manicminer

Summary
I have a project with a README.md, which contains a relative link to the tree view of the project, in the form /group/project/tree/master. Since updating from 7.7.2 to 7.8.0, the link is rendered incorrectly as /group/project/blob/master/group/project/tree/master.

To reproduce
Create a project with README.md containing:

[Click here](/group/project/tree/master) to browse the project

Expected result
Clicking the link should take me to http://gitlab.hostname/group/project/tree/master. This was the case with Gitlab 7.7.2

Actual result
Clicking the link takes me to http://gitlab.hostname/group/project/blob/master/group/project/tree/master

GitLab version
GitLab 7.8.0 bcc8dbd Installed with gitlab_7.8.0-omnibus-1_amd64.deb

Output of rake gitlab:check

Checking Environment ...

Git configured for git user? ... yes

Checking Environment ... Finished

Checking GitLab Shell ...

GitLab Shell version >= 2.5.3 ? ... OK (2.5.3)
Repo base directory exists? ... yes
Repo base directory is a symlink? ... no
Repo base owned by git:git? ... yes
Repo base access is drwxrws---? ... yes
Satellites access is drwxr-x---? ... yes
hooks directories in repos are links: ...
2/1 ... ok
4/2 ... ok
2/4 ... ok
2/5 ... ok
2/6 ... ok
2/7 ... ok
5/8 ... ok
6/9 ... ok
2/10 ... ok
2/11 ... ok
2/12 ... ok
5/13 ... ok
2/14 ... ok
3/15 ... ok
3/16 ... ok
3/17 ... ok
2/18 ... ok
3/19 ... ok
2/39 ... ok
5/40 ... ok
2/41 ... ok
2/43 ... ok
2/44 ... ok
2/45 ... ok
2/47 ... ok
3/48 ... repository is empty
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Check directories and files:
    /var/opt/gitlab/git-data/repositories: OK
    /var/opt/gitlab/.ssh/authorized_keys: OK
Test redis-cli executable: redis-cli 2.8.2
Send ping to redis server: PONG
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Sidekiq ...

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

Checking Sidekiq ... Finished

Checking LDAP ...

LDAP is disabled in config/gitlab.yml

Checking LDAP ... Finished

Checking GitLab ...

Database config exists? ... yes
Database is SQLite ... no
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... 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: ...
2/1 ... yes
4/2 ... yes
2/4 ... yes
2/5 ... yes
2/6 ... yes
2/7 ... yes
5/8 ... yes
6/9 ... yes
2/10 ... yes
2/11 ... yes
2/12 ... yes
5/13 ... yes
2/14 ... yes
3/15 ... yes
3/16 ... yes
3/17 ... yes
2/18 ... yes
3/19 ... yes
2/39 ... yes
5/40 ... yes
2/41 ... yes
2/43 ... yes
2/44 ... yes
2/45 ... yes
2/47 ... yes
3/48 ... yes
Projects have satellites? ...
2/1 ... yes
4/2 ... yes
2/4 ... yes
2/5 ... yes
2/6 ... yes
2/7 ... yes
5/8 ... yes
6/9 ... yes
2/10 ... yes
2/11 ... yes
2/12 ... yes
5/13 ... yes
2/14 ... yes
3/15 ... yes
3/16 ... yes
3/17 ... yes
2/18 ... yes
3/19 ... yes
2/39 ... yes
5/40 ... yes
2/41 ... yes
2/43 ... yes
2/44 ... yes
2/45 ... yes
2/47 ... yes
3/48 ... yes
Redis version >= 2.0.0? ... yes
Ruby version >= 2.0.0 ? ... yes (2.1.5)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 1.7.10 ? ... yes (2.0.5)

Checking GitLab ... Finished