Fixup command names in some usage strings.

Most usage strings, such as for command xxx, start with "git-xxx".
This updates the rebels to conform to the general pattern.
(The git wrapper is an exception to this, of course ...)

Signed-off-by: Ramsay Allan Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>

mktree.c

@@ -71,7 +71,7 @@ static void write_tree(unsigned char *sha1)
 	write_sha1_file(buffer, offset, tree_type, sha1);
 }
 
-static const char mktree_usage[] = "mktree [-z]";
+static const char mktree_usage[] = "git-mktree [-z]";
 
 int main(int ac, char **av)
 {