[HipHop] Porting wikidiff2 to hhvm

[Max Semenik]

Hey, I took a stab at making wikidiff2 a HHVM extension: [1]
Seems to work fine, however I wonder if there is really no memory
pools available in HHVM that would allow to mitigate possible leaks
or sandbox consumption?  Iif you simply grow HPHP::String infinitely
it will eat whole swap before segfaulting, which seems like a
regression if compared to PHP.

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[1] https://gerrit.wikimedia.org/r/117362

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Best regards,
  Max Semenik ([[User:MaxSem]])