test-lib: try harder to ensure a working jgit

The JGIT prereq uses `type jgit` to determine whether jgit is present.
While this is usually sufficient, it won't help if the jgit found is
badly broken.  This wastes time running tests which fail due to no fault
of our own.

Use `jgit --version` instead, to guard against cases where jgit is
present on the system, but will fail to run, e.g. because of some JRE
issue, or missing Java dependencies.  Checking that it gets far enough
to process the '--version' argument isn't perfect, but seems to be good
enough in practice.  It's also consistent with how we detect some other
dependencies, see e.g. the CURL and UNZIP prerequisites.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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