A polymoprh zap which creates a long worm can hit and transform the
same monster again depending upon tail segment placement. Similar
behavior occurs if monpolycontrol is set in wizard mode and player
chooses 'long worm' for what to transform an existing one into (in
which case polymorph fails and zap might hit that same worm again
in another segment, prompting player to choose its new shape again).
Simplest fix would be to make tail segments be immune to polymorph,
but that would prevent players from deliberately attacking the tail
(for polymorph attacks only). Next simplest would be to make long
worms M2_NOPOLY so that polymorph can't create them, then just live
with multiple promptings when monpolycontrol is set. This fix
tracks whether a long worm has just been created via polymorph (or
explicitly retained its shape via monpolycontrol) and makes further
hits on same creature on same zap have no effect. It does so by
setting mon->mextra->mcorpsenm to PM_LONG_WORM when a long worm is
result of polymorph, and setting context.bypasses to get end-of-zap
cleanup. (It doesn't bother discarding mon->mextra if reset of
mcorpsenm leaves mextra empty.)
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