Enable passing java options to jgit command line executable

This allows to pass java options to the jgit command line executable
by setting the environment variable "java_args", e.g.

$ java_args="-Xmx8g" jgit log

Change-Id: Ic7271b104737a8306d9db0f1895079b9f7bb7fd0
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.pgm/jgit.sh

@@ -79,7 +79,7 @@ CYGWIN*)
 Darwin)
 	if [ -e /System/Library/Frameworks/JavaVM.framework ]
 	then
-		java_args='
+		java_args+='
 			-Dcom.apple.mrj.application.apple.menu.about.name=JGit
 			-Dcom.apple.mrj.application.growbox.intrudes=false
 			-Dapple.laf.useScreenMenuBar=true