filemerge: remove `uselocalpath` argument from `_maketempfiles()`
The `localpath` argument is unused if `uselocalpath` is false, so we
can use `None` as a sentinel value for the variable instead and remove
extra `uselocalpath` argument. That's not much of a win, but it's a
small step towards further improvements.
Differential Revision: https://phab.mercurial-scm.org/D12189
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