Fix compiler warnings in DiffFormatter.writeGitLinkText

- Remove declaration of IOException that is no longer thrown

- Add missing //$NON-NLS-1$ to prevent "Non-externalized string literal"
  warning.

These warnings seem to have been introduced by If13f7b406.

Change-Id: I30058eed31b92067a6ab22e787732b08e29f8d63
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java

@@ -665,10 +665,9 @@ public class DiffFormatter implements AutoCloseable {
 		format(res.header, res.a, res.b);
 	}
 
-	private static byte[] writeGitLinkText(AbbreviatedObjectId id)
-		throws IOException {
+	private static byte[] writeGitLinkText(AbbreviatedObjectId id) {
 		return encodeASCII("Subproject commit " + id.name() //$NON-NLS-1$
-				+ "\n");
+				+ "\n"); //$NON-NLS-1$
 	}
 
 	private String format(AbbreviatedObjectId id) {