When we compare the number of bytes produced with the offset for a
backreference, make the signedness of the bytes produced clear,
by sticking it into a size_t. This avoids a signed/unsigned compare
warning from MSVC (public issue 71), and also is slightly clearer.

Since the line is now so long the explanatory comment about the -1u
trick has to go somewhere else anyway, I used the opportunity to
explain it in slightly more detail.

This is a purely stylistic change; the emitted assembler from GCC
is identical.

R=jeff