Externalize error message in FileRepository#convertRefStorage

Change-Id: I5d3acaa883ee7eb8d244b55b3e708cbe018690c6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties

 unknownObjectInIndex=unknown object {0} found in index but not in pack file
 unknownObjectType=Unknown object type {0}.
 unknownObjectType2=unknown
+unknownRefStorageFormat=Unknown ref storage format "{0}"
 unknownRepositoryFormat=Unknown repository format
 unknownRepositoryFormat2=Unknown repository format "{0}"; expected "0".
 unknownTransportCommand=unknown command {0}

org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java

 	/***/ public String unknownObjectInIndex;
 	/***/ public String unknownObjectType;
 	/***/ public String unknownObjectType2;
+	/***/ public String unknownRefStorageFormat;
 	/***/ public String unknownRepositoryFormat;
 	/***/ public String unknownRepositoryFormat2;
 	/***/ public String unknownTransportCommand;

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java

 	 * @throws IOException
 	 *             on I/O problems.
 	 */
-	@SuppressWarnings("nls")
 	public void convertRefStorage(String format, boolean writeLogs,
 			boolean backup) throws IOException {
 		if (format.equals("reftable")) { //$NON-NLS-1$
 				convertToPackedRefs(backup);
 			}
 		} else {
-			throw new IOException(String.format(
-					"unknown supported ref storage format '%s'", format));
+			throw new IOException(MessageFormat
+					.format(JGitText.get().unknownRefStorageFormat, format));
 		}
 	}
 }