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>

mktag.c

@@ -123,7 +123,7 @@ int main(int argc, char **argv)
 	unsigned char result_sha1[20];
 
 	if (argc != 1)
-		usage("cat <signaturefile> | git-mktag");
+		usage("git-mktag < signaturefile");
 
 	setup_git_directory();