Fix crash deep inside VcsOverlayProxyModel::data()

Summary:
VcsOverlayProxyModel::data() calls ProjectController::changesModel()
which currently is a non-const method which might create an instance
of ProjectChangesModel which in turn triggers some KJobs.

Avoid that by instantiating ProjectChangesModel() immediately during
startup. I don't think that has a real performance impact.

BUG: 384162
FIXED-IN: 5.2.1

Reviewers: apol

Reviewed By: apol

Subscribers: mwolff, dfaure, brauch, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8852