Jenkins: Set Phabricator as git repository browser

Here comes the counter part of what has been implemented in !30 (closed).

It enables Jenkins to link back to Phabricator web interface in order to easily browse commits, files history or even diffs.

You can see the results on the last Jenkins jobs building the master branch of swh-web (Jenkins pipeline configuration has been hotpatched):

• https://jenkins.softwareheritage.org/job/DWAPPS/job/tests/723/ (notice the phabricator link in the Changes section)
• https://jenkins.softwareheritage.org/job/DWAPPS/job/tests/723/changes

And this time I have tested my changes using tox ;-)

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Migrated from D1834 (view on Phabricator)