Make FS OS X detection work for OpenJDK

OpenJDK sets the os.name system.property to "Darwin", while Apple's
version says "Mac OS X".

Change-Id: If08f8e7b8ef94ec00023a3f78bbf502ebd9699fb

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -1015,10 +1015,9 @@ public class DirCacheCheckout {
 	}
 
 	private static boolean isValidPathSegment(CanonicalTreeParser t) {
-		boolean isWindows = "Windows".equals(SystemReader.getInstance()
+		String osName = SystemReader.getInstance().getProperty("os.name");
-				.getProperty("os.name"));
+		boolean isWindows = "Windows".equals(osName);
-		boolean isOSX = "Mac OS X".equals(SystemReader.getInstance()
+		boolean isOSX = "Darwin".equals(osName) || "Mac OS X".equals(osName);
-				.getProperty("os.name"));
 		boolean ignCase = isOSX || isWindows;
 
 		int ptr = t.getNameOffset();

org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java

@@ -105,6 +105,6 @@ abstract class FS_POSIX extends FS {
 						return System.getProperty("os.name");
 					}
 				});
-		return "Mac OS X".equals(osDotName);
+		return "Mac OS X".equals(osDotName) || "Darwin".equals(osDotName);
 	}
 }