Make default expiration period of reflog used for stash infinite

This makes the default expiration period for the reflog that implements
stash infinite.

The original behaviour to autoexpire old stashes can be restored by using
the gc.refs/stash.{reflogexpire,reflogexpireunreachable} configration
variables introduced by the previous commit.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-reflog.c

@@ -441,6 +441,17 @@ static void set_reflog_expiry_param(struct cmd_reflog_expire_cb *cb, int slot, c
 		}
 	}
 
+	/*
+	 * If unconfigured, make stash never expire
+	 */
+	if (!strcmp(ref, "refs/stash")) {
+		if (!(slot & EXPIRE_TOTAL))
+			cb->expire_total = 0;
+		if (!(slot & EXPIRE_UNREACH))
+			cb->expire_unreachable = 0;
+		return;
+	}
+
 	/* Nothing matched -- use the default value */
 	if (!(slot & EXPIRE_TOTAL))
 		cb->expire_total = default_reflog_expire;