xdiff: remove xemit related logic xemit handles "diff formatting and output" with options like context lines, whether show function names, etc. That is handled more cleanly at a higher level in hg. Removing context line parameters would also make the trimming logic (D2686) cleaner and more confident. See [1]. [1]: https://github.com/git/git/commit/d2f82950a9226ae1102a7a97f03440a4bf8c6c09 Differential Revision: https://phab.mercurial-scm.org/D2705

xdiff: remove unused structure, functions, and constants `bdiffparam_t` is unused. `xdl_fall_back_diff` is no longer used after D2573. `XDL_MMB_READONLY`, `XDL_MMF_ATOMIC` are unused. `XDL_BDOP*` are unused since there is no xdiff binary diff algorithm. `anchors` feature is not used. It's also relatively new in git. Differential Revision: https://phab.mercurial-scm.org/D2684

xdiff: remove whitespace related feature In Mercurial, whitespace related handling are done at a higher level than the low-level diff algorithm so "ignore spaces". So it's not used by mdiff. Some of the upcoming optimizations would be more difficult with whitespace related features kept. So let's remove them. Differential Revision: https://phab.mercurial-scm.org/D2683