GitDateParser#ParseableSimpleDateFormat: Make formatStr private final

ParseableSimpleDateFormat is an enum, and enums must be immutable,
hence the member should be final. At the same time, make it private
since it does not need to be publicly visible.

Change-Id: I7e181f591038d556f1123b6e37adf8441059e99a
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

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

@@ -126,7 +126,7 @@ public class GitDateParser {
 		DEFAULT("EEE MMM dd HH:mm:ss yyyy Z"), // //$NON-NLS-1$
 		LOCAL("EEE MMM dd HH:mm:ss yyyy"); //$NON-NLS-1$
 
-		String formatStr;
+		private final String formatStr;
 
 		private ParseableSimpleDateFormat(String formatStr) {
 			this.formatStr = formatStr;