With multiple jobs running in parallel, baseline could be initialized in
parallel and threads may see partly initialized baselines too. That
should be avoided.

Change-Id: Idb0d99e1071e957edd86bfb17480d62fc0a6baf1
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/189559
Tested-by: PDE Bot <pde-bot@eclipse.org>