config: reject bogus values for core.checkstat

If you feed nonsense config like:

  git -c core.checkstat=foobar status

we'll silently ignore the unknown value, rather than reporting an error.
This goes all the way back to c08e4d5b5c (Enable minimal stat checking,
2013-01-22).

Detecting and complaining now is technically a backwards-incompatible
change, but I don't think anybody has any reason to use an invalid value
here. There are no historical values we'd want to allow for backwards
compatibility or anything like that. We are better off loudly telling
the user that their config may not be doing what they expect.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1392,6 +1392,9 @@ static int git_default_core_config(const char *var, const char *value,
 			check_stat = 1;
 		else if (!strcasecmp(value, "minimal"))
 			check_stat = 0;
+		else
+			return error(_("invalid value for '%s': '%s'"),
+				     var, value);
 	}
 
 	if (!strcmp(var, "core.quotepath")) {