fix #K3474 - "you detect it where <foo> was"

A vampire in bat form was seen via infravision or possibly telepathy,
then when it changed into fog cloud form the feedback was
|You now detect it where the vampire bat was.
The message substitutes "detect" for "see" when the new form can't
be seen and the monster name formatting yields "it" for that case.

Give a vanish message instead since that is effectively what happens.

src/monmove.c

@@ -1907,6 +1907,7 @@ vamp_shift(
 {
     int reslt = 0;
     char oldmtype[BUFSZ];
+    boolean sawmon = canseemon(mon); /* before shape change */
 
     /* remember current monster type before shapechange */
     Strcpy(oldmtype, domsg ? noname_monnam(mon, ARTICLE_THE) : "");
@@ -1920,9 +1921,14 @@ vamp_shift(
     }
 
     if (reslt && domsg) {
-        pline("You %s %s where %s was.",
-              !canseemon(mon) ? "now detect" : "observe",
-              noname_monnam(mon, ARTICLE_A), oldmtype);
+        /* might have seen vampire/bat/wolf with infravision then be
+           unable to see the same creature when it turns into a fog cloud */
+        if (canspotmon(mon))
+            You("%s %s where %s was.",
+                !canseemon(mon) ? "now detect" : "observe",
+                noname_monnam(mon, ARTICLE_A), oldmtype);
+        else
+            You("can no longer %s %s.", sawmon ? "see" : "sense", oldmtype);
         /* this message is given when it turns into a fog cloud
            in order to move under a closed door */
         display_nhwindow(WIN_MESSAGE, FALSE);