show-branch: match documentation and usage

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-show-branch.txt

@@ -52,6 +52,11 @@ OPTIONS
         appear in topological order (i.e., descendant commits
         are shown before their parents).
 
+--sparse::
+	By default, the output omits merges that are reachable
+	from only one tip being shown.  This option makes them
+	visible.
+
 --more=<n>::
 	Usually the command stops output upon showing the commit
 	that is the common ancestor of all the branches.  This

builtin-show-branch.c

@@ -6,7 +6,7 @@
 #include "builtin.h"
 
 static const char show_branch_usage[] =
-"git-show-branch [--dense] [--current] [--all] [--heads] [--tags] [--topo-order] [--more=count | --list | --independent | --merge-base ] [--topics] [<refs>...]";
+"git-show-branch [--sparse] [--current] [--all] [--heads] [--tags] [--topo-order] [--more=count | --list | --independent | --merge-base ] [--topics] [<refs>...]";
 
 static int default_num = 0;
 static int default_alloc = 0;