[Rd] [patch] Support many columns in model.matrix

[Karl Millar]

Generating a model matrix with very large numbers of columns overflows
the stack and/or runs very slowly, due to the implementation of
TrimRepeats().

This patch modifies it to use Rf_duplicated() to find the duplicates.
This makes the running time linear in the number of columns and
eliminates the recursive function calls.

Thanks
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