Const tightening.

Mark Wooding noticed there was a type mismatch warning in git.c; this
patch does things slightly differently (mostly tightening const) and
was what I was holding onto, waiting for the setup-revisions change
to be merged into the master branch.

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

shell.c

@@ -15,7 +15,7 @@ static int do_generic_cmd(const char *me, char *arg)
 	my_argv[1] = arg;
 	my_argv[2] = NULL;
 
-	return execv_git_cmd((char**) my_argv);
+	return execv_git_cmd(my_argv);
 }
 
 static struct commands {