Bitbucket: contributed buttons have no hover title

Created by: lguychard

Steps to reproduce:

1. Visit a Bitbucket server instance with the browser extension or native integration activated (eg. bitbucket.sgdev.org, which has the native integration)
2. Hover buttons contributed by Sourcegraph (eg. view repo, view file...)

Expected behavior:

A helpful hover title is displayed (eg. "View repository on Sourcegraph", "View file on Sourcegraph")

Actual behavior:

No hover title is displayed:

This is an issue because it's unclear to a new user what these buttons do, since they only show the Sourcegraph logo.