Set default file encoding used for JGit tests to UTF-8

Set default file encoding used for JGit tests to UTF-8

This patch fixes the problem that JGit tests run from Maven fail on
Mac OS X [1]. In Eclipse the tests succeed since we set Eclipse
workspace encoding to UTF-8 via "Preferences > General > Workspace
> Text file encoding", checked via JConsole that this setting changes
the JVM system property of the test run. This change copies this
setting to the Maven test environment so that we get consistent test
results on all platforms.

[1] http://kerneltrap.org/mailarchive/git/2009/2/6/4916414/thread

Change-Id: Ia657d4037c518411f7bc3c75626dbafb1f4706a2
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>