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

xdiff: use int64 for hash table size Follow-up of the previous "long" -> "int64" change. Now xdiff only uses int for return values and small integers (ex. booleans, shifting score, bits in hash table size, etc) so it should be able to handle large input. Differential Revision: https://phab.mercurial-scm.org/D2765

xdiff: remove unused xpp and xecfg parameters They are unused. Thus removed. Differential Revision: https://phab.mercurial-scm.org/D2764