AC_Fence: emit a FENCE_STATUS message if we newly-breach the fences

This should resolve a problem in autotest where we don't detect the
fence as being breached as ArduPilot never announces the fact it has
breached - it just changes mode to RTL but the interval on the
FENCE_STATUS message never aligns with the time the vehicle is breached.

libraries/AC_Fence/AC_Fence.cpp

@@ -401,7 +401,14 @@ void AC_Fence::record_breach(uint8_t fence_type)
 {
     // if we haven't already breached a limit, update the breach time
     if (!_breached_fences) {
-        _breach_time = AP_HAL::millis();
+        const uint32_t now = AP_HAL::millis();
+        _breach_time = now;
+
+        // emit a message indicated we're newly-breached, but not too often
+        if (now - _last_breach_notify_sent_ms > 1000) {
+            _last_breach_notify_sent_ms = now;
+            gcs().send_message(MSG_FENCE_STATUS);
+        }
     }
 
     // update breach count

libraries/AC_Fence/AC_Fence.h

@@ -168,9 +168,11 @@ private:
     uint8_t         _breached_fences;       // bitmask holding the fence type that was breached (i.e. AC_FENCE_TYPE_ALT_MIN, AC_FENCE_TYPE_CIRCLE)
     uint32_t        _breach_time;           // time of last breach in milliseconds
     uint16_t        _breach_count;          // number of times we have breached the fence
+    uint32_t _last_breach_notify_sent_ms;  // last time we sent a message about newly-breaching the fences
 
     uint32_t        _manual_recovery_start_ms;  // system time in milliseconds that pilot re-took manual control
 
+
     AC_PolyFence_loader _poly_loader{_total}; // polygon fence
 };