diff: add option metaVar for --src-prefix and --dst-prefix

Fixes exception if using "jgit diff --help".

Bug: 544735
Change-Id: I694ff3103da4bc199dd03c40962c5be191eddcd1
Signed-off-by: Andre Bossert <andre.bossert@siemens.com>

org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties

@@ -133,6 +133,7 @@ metaVar_path=path
 metaVar_paths=path ...
 metaVar_pattern=pattern
 metaVar_port=PORT
+metaVar_prefix=PREFIX
 metaVar_ref=REF
 metaVar_refs=REFS
 metaVar_refspec=refspec

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java

@@ -151,12 +151,12 @@ class Diff extends TextBuiltin {
 		diffFmt.setAbbreviationLength(OBJECT_ID_STRING_LENGTH);
 	}
 
-	@Option(name = "--src-prefix", usage = "usage_srcPrefix")
+	@Option(name = "--src-prefix", metaVar = "metaVar_prefix", usage = "usage_srcPrefix")
 	void sourcePrefix(String path) {
 		diffFmt.setOldPrefix(path);
 	}
 
-	@Option(name = "--dst-prefix", usage = "usage_dstPrefix")
+	@Option(name = "--dst-prefix", metaVar = "metaVar_prefix", usage = "usage_dstPrefix")
 	void dstPrefix(String path) {
 		diffFmt.setNewPrefix(path);
 	}