When using ngettext (and its variants), the generated code would be

wrong if both strings ended up using its own interpolated arguments. For

instance, this code:

N_(`This is ${var1}`,
   `This is ${var2}`,
   2)


... would end up generating the right strings, but the wrong arguments

to pass in. The way we were building up the merged list of variables was

wrong.
This change fixes this by introducing a new merging function for these

lists of variables, which goes through the lists and builds a merged,

deduplicated list of variables that can be passed in.
The unit test coverage has been expanded to include unique variables in

the ngettext-related tests that involve interpolation, and to test

both singular and plural variants in all tests.

Unit tests pass.
Verified this fixed a bug I encountered when testing /r/11022.