FIX: deep-link into individual commit for non-root URLs.

Use now the gerrit.canonicalWebUrl for allowing the
correct composition of deep-links from individual
points in the branch-network.

Previously the (wrong) assumption that '/' is the
main context was wrong and lead to broken deep-links.

Change-Id: Id703a83875c259ea3eebb4585cec5667d2bf8346
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>