Gerrit: Use magic branch options to specify reviewers

Enables specifying reviewers over http[s].

Supported since gerrit 2.6 (specifying reviewers will be broken for
earlier versions).

Prepare for supporting other options (draft, publish, topic...)

Change-Id: Id0b6508409123cddb9dec5ea9e78d7a6a1423057
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Gerrit: Display dependent changes as tree.

Add a depth member to GerritChange indicating the tree depth
and populate it by searches over the dependent changes.
Display the root changes sorted by date and add dependent changes
to their deph=1 parent, avoiding deeply nested items.

Change-Id: I66ccf392ce47514841c661820797e81996e9e5cc
Reviewed-by: Orgad Shaneh <orgads@gmail.com>