config: Give error message when not changing a multivar

When trying to set a multivar with "git config var value", "git config"
issues

warning: remote.repoor.push has multiple values

leaving the user under the impression that the operation succeeded,
unless one checks the return value.

Instead, make it

warning: remote.repoor.push has multiple values
error: cannot overwrite multiple values with a single value
       Use a regexp, --add or --set-all to change remote.repoor.push.

to be clear and helpful.

Note: The "warning" is raised through other code paths also so that it
needs to remain a warning for these (which do not raise the error). Only
the caller can determine how to go on from that.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -436,9 +436,14 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 			      NULL, NULL);
 	}
 	else if (actions == ACTION_SET) {
+		int ret;
 		check_argc(argc, 2, 2);
 		value = normalize_value(argv[0], argv[1]);
-		return git_config_set(argv[0], value);
+		ret = git_config_set(argv[0], value);
+		if (ret == CONFIG_NOTHING_SET)
+			error("cannot overwrite multiple values with a single value\n"
+			"       Use a regexp, --add or --set-all to change %s.", argv[0]);
+		return ret;
 	}
 	else if (actions == ACTION_SET_ALL) {
 		check_argc(argc, 2, 3);