[PATCH] Fix escaping for '\' and '"' in pageinspect for gist

Hi hackers!

I noticed, that there is bug in escaping values that contains '\' or '"' in text representation
inside pageinspect for gist: the string 'foo"bar' are printed like "foo""bar" and not "foo\"bar".

To fix that, we should do appendStringInfoCharMacro(&buf, '\\'); instead of
appendStringInfoCharMacro(&buf, ch); in case ch is one of that symbols.

Any thoughts?

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Best regards,
Roman Khapov