fix issue #1377 - Forcefight vs displacer beasts

Issue reported by elunna:  Using the 'F' prefix against a displacer
beast prevented swapping places.

This doesn't use the suggested fix.  It is quite short but there is
a large diff due to change in indentation and reformatting several
comments because of that.

Attacking a displacer beast either with or without 'F' might miss,
hit, or swap places.  It won't "harmlessly attack thin air."

Fixes #1377

doc/fixes3-7-0.txt

@@ -2098,6 +2098,8 @@ when walking into/against a locked closed door, 'autounlock'==kick didn't
 having a 1% chance of creating rideable monsters with worn saddle gave knights
 	a 1% chance of creating an extra saddle for starting pony; it wasn't
 	tracked with other objects so produced a trivial memory leak
+hen attacking a displacer beast, using the 'F' forcefight prefix prevented it
+	from swapping places with the hero
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

src/hack.c

@@ -1956,10 +1956,12 @@ domove_attackmon_at(
                                    && bad_rock(gy.youmonst.data, x, u.uy0))))
                       && goodpos(u.ux0, u.uy0, mtmp, GP_ALLOW_U));
         /* if not displacing, try to attack; note that it might evade;
-           also, we don't attack tame when _safepet_ */
+           also, we don't attack tame or peaceful when safemon() */
-        if (!*displaceu && do_attack(mtmp))
+        if (!*displaceu) {
+            if (do_attack(mtmp))
                 return TRUE;
         }
+    }
     return FALSE;
 }
 
@@ -2696,6 +2698,8 @@ domove_core(void)
             return;
     }
 
+    if (!displaceu) {
+
         if (domove_fight_ironbars(x, y))
             return;
 
@@ -2718,9 +2722,9 @@ domove_core(void)
         /* treat entering a visible gas cloud region like entering a trap;
            there could be a known trap as well as a region at the target spot;
            if so, ask about entring the region first; even though this could
-       lead to two consecutive confirmation prompts, the situation seems to
+           lead to two consecutive confirmation prompts, the situation seems
-       be too uncommon to warrant a separate case with combined trap+region
+           to be too uncommon to warrant a separate case with combined
-       confirmation */
+           trap+region confirmation */
         if (ParanoidTrap && !Blind && !Stunned && !Confusion && !Hallucination
             /* skip if player used 'm' prefix or is moving recklessly */
             && (!svc.context.nopick || svc.context.run)
@@ -2729,12 +2733,14 @@ domove_core(void)
             && ((oldreg = visible_region_at(u.ux, u.uy)) == 0
                 /* if moving from one region into another, only ask for
                    confirmation if the one potentially being entered inflicts
-               damage (poison gas) and the one being exited doesn't (vapor) */
+                   damage (poison gas) and the one being exited doesn't
+                   (vapor) */
                 || (reg_damg(newreg) > 0 && reg_damg(oldreg) == 0))
             /* check whether attempted move will be viable */
             && test_move(u.ux, u.uy, u.dx, u.dy, TEST_MOVE)
-        /* we don't override confirmation for poison resistance since the
+            /* we don't override confirmation for poison resistance since
-           region also hinders hero's vision even if/when no damage is done */
+               the region also hinders hero's vision even if/when no damage
+               is done */
             ) {
             char qbuf[QBUFSZ];
 
@@ -2747,18 +2753,13 @@ domove_core(void)
                 return;
             }
         }
-    /* maybe ask player for confirmation before walking into known traps */
+        /* maybe ask player for confirmation before walking into known trap */
         if (ParanoidTrap && !Stunned && !Confusion
             /* skip if player used 'm' prefix or is moving recklessly */
             && (!svc.context.nopick || svc.context.run)
             /* check for discovered trap */
             && (trap = t_at(x, y)) != 0 && trap->tseen
             /* check whether attempted move will be viable */
-    /*
-     * FIXME:
-     *  this will result in "Really step into trap?" if there is a
-     *  peaceful or tame monster already there.
-     */
             && test_move(u.ux, u.uy, u.dx, u.dy, TEST_MOVE)
             /* override confirmation if the trap is harmless to the hero */
             && (immune_to_trap(&gy.youmonst, trap->ttyp) != TRAP_CLEARLY_IMMUNE
@@ -2766,7 +2767,8 @@ domove_core(void)
                    hero can't tell what they are, so treat as dangerous */
                 || Hallucination)) {
             char qbuf[QBUFSZ];
-        int traptype = (Hallucination ? rnd(TRAPNUM - 1) : (int) trap->ttyp);
+            int traptype = (Hallucination ? rnd(TRAPNUM - 1)
+                                          : (int) trap->ttyp);
             boolean into = into_vs_onto(traptype);
 
             Snprintf(qbuf, sizeof qbuf, "Really %s %s that %s?",
@@ -2783,14 +2785,15 @@ domove_core(void)
             }
         }
 
-    if (u.utrap) {
+        if (u.utrap) { /* when u.utrap is True, displaceu is False */
             boolean moved = trapmove(x, y, trap);
 
             if (!u.utrap) {
                 disp.botl = TRUE;
                 reset_utrap(TRUE); /* might resume levitation or flight */
             }
-        /* might not have escaped, or did escape but remain in same spot */
+            /* might not have escaped, or did escape but remain in the same
+               spot */
             if (!moved)
                 return;
         }
@@ -2810,6 +2813,8 @@ domove_core(void)
             return;
         }
 
+    } /* !dislacedu */
+
     /* Move ball and chain.  */
     if (Punished)
         if (!drag_ball(x, y, &bc_control, &ballx, &bally, &chainx, &chainy,