config: use the_hash_algo in abbrev comparison

Switch one use of a hard-coded 40 constant to use the_hash_algo.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1204,7 +1204,7 @@ static int git_default_core_config(const char *var, const char *value, void *cb)
 			default_abbrev = -1;
 		else {
 			int abbrev = git_config_int(var, value);
-			if (abbrev < minimum_abbrev || abbrev > 40)
+			if (abbrev < minimum_abbrev || abbrev > the_hash_algo->hexsz)
 				return error(_("abbrev length out of range: %d"), abbrev);
 			default_abbrev = abbrev;
 		}