xdiff: add a bdiff hunk mode xdiff generated hunks for the differences (ex. questionmarks in the `@@ -?,? +?,? @@` part from `diff --git` output). However, bdiff generates matched hunks instead. This patch adds a `XDL_EMIT_BDIFFHUNK` flag used by the output function `xdl_call_hunk_func`. Once set, xdiff will generate bdiff-like hunks instead. That makes it easier to use xdiff as a drop-in replacement of bdiff. Note that since `bdiff('', '')` returns `[(0, 0, 0, 0)]`, the shortcut path `if (xscr)` is removed. I have checked functions called with `xscr` argument (`xdl_mark_ignorable`, `xdl_call_hunk_func`, `xdl_emit_diff`, `xdl_free_script`) work just fine with `xscr = NULL`. Test Plan: Will be tested in a later patch. Differential Revision: https://phab.mercurial-scm.org/D2575