Gerrit links the git commits in various places (each patchset in the changeset review screen, the tips of the branches in the branch administration screen etc) to some repository browser (which can be very useful to see the state of files which are not touched by a changeset, for example). That repository browser used to be gitblit, but has been recently changed to be Differential. Unfortunately, it does not really work.

More specifically,

• links to commits which have not been merged do not work. (I.e. in a merged changeset only the last commit has a working link, and in open changesets none do.)
• links in a project's Access/Branches menu never work (with a different error message).

Example not working review link: https://gerrit.wikimedia.org/r/#/c/258711/ PS6 -> https://phabricator.wikimedia.org/rMWVAf41d6e47997ea8a7849292ec78a185a252c8627a
Example not working branch link: https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/skins/Vector,branches master -> https://phabricator.wikimedia.org/diffusion/SVEC/history/HEAD/

Especially the link to the last commit of an open changeset is important for code review. Please revert these links back to gitblit until kinks in Differential have been worked out.