diff: Fix bad metaVar reference in --abbrev option

Change-Id: If92545b731ff80bff071aee9bbd852bbd187c7c5
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java
index ebdb74f..2be5722 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java
@@ -122,7 +122,7 @@ void unified(int lines) {
 		diffFmt.setContext(lines);
 	}
 
-	@Option(name = "--abbrev", metaVar = "n")
+	@Option(name = "--abbrev", metaVar = "metaVar_n")
 	void abbrev(int lines) {
 		diffFmt.setAbbreviationLength(lines);
 	}