xdiff: resolve signed unsigned comparison warning
Since the value won't be changed inside the code (because context lines
feature was removed by D2705), let's just remove the variable and inline
the 0 value.
The code might be potentially further simplified. But I'd like to make sure
correctness is easily verifiable in this patch.
Differential Revision: https://phab.mercurial-scm.org/D2766
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