Fix help text for option --enable of command debug-rebuild-ref-tree

This fixes a MissingResourceException thrown when executing
jgit debug-rebuild-ref-tree --help

Change-Id: I637ea55084a913f5105ebf4cf2baef8b81877938
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -244,6 +244,7 @@ usage_MergesTwoDevelopmentHistories=Merges two development histories
 usage_ReadDirCache= Read the DirCache 100 times
 usage_RebuildCommitGraph=Recreate a repository from another one's commit graph
 usage_RebuildRefTree=Copy references into a RefTree
+usage_RebuildRefTreeEnable=set extensions.refsStorage = reftree
 usage_Remote=Manage set of tracked repositories
 usage_RepositoryToReadFrom=Repository to read from
 usage_RepositoryToReceiveInto=Repository to receive into

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/debug/RebuildRefTree.java

@@ -67,7 +67,7 @@ import org.kohsuke.args4j.Option;
 
 @Command(usage = "usage_RebuildRefTree")
 class RebuildRefTree extends TextBuiltin {
-	@Option(name = "--enable", usage = "set extensions.refsStorage = reftree")
+	@Option(name = "--enable", usage = "usage_RebuildRefTreeEnable")
 	boolean enable;
 
 	private String txnNamespace;