Expose org.eclipse.jgit.junit via jgit p2 repository

EGit Tycho builds on build.eclipse.org frequently hit corrupted artifacts
which leads to broken builds. Cleaning up these corrupted files is tedious
since it requires file system access on the build server. Hence we want to
switch to use job-local m2 repositories. This requires that build artifacts
are shared between the jgit and egit build jobs via p2. Therefore the
bundle org.eclipse.jgit.junit needs to be exposed via p2 repository.

Change-Id: I0ccd7763eede117cb68240fdd25f13d6e6f6a2c1
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>