ci: install JGit dependency

ci: install JGit dependency

We have some tests in t5310 that use JGit to verify that bitmaps can be
read both by Git and by JGit. We do not execute these tests in our CI
jobs though because we don't make JGit available there. Consequently,
the tests basically bitrot because almost nobody is ever going to have
JGit in their path.

Install JGit to plug this test gap.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>