Record the command invocation path early

We will need the command invocation path in system_path(). This path was
passed to setup_path(), but  system_path() can be called earlier, for
example via:

    main
      commit_pager_choice
        setup_pager
          git_config
            git_etc_gitconfig
              system_path

Therefore, we introduce git_set_argv0_path() and call it as soon as
possible.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

shell.c

@@ -15,7 +15,7 @@ static int do_generic_cmd(const char *me, char *arg)
 {
 	const char *my_argv[4];
 
-	setup_path(NULL);
+	setup_path();
 	if (!arg || !(arg = sq_dequote(arg)))
 		die("bad argument");
 	if (prefixcmp(me, "git-"))
@@ -37,7 +37,7 @@ static int do_cvs_cmd(const char *me, char *arg)
 	if (!arg || strcmp(arg, "server"))
 		die("git-cvsserver only handles server: %s", arg);
 
-	setup_path(NULL);
+	setup_path();
 	return execv_git_cmd(cvsserver_argv);
 }