bdiff: add a xdiffblocks method This is similar to `bdiff.blocks`, but uses xdiff as the backend. The indent heuristic is turned on by default since it has little overhead and improves diff quality significantly. Differential Revision: https://phab.mercurial-scm.org/D2602

xdiff: reduce indent heuristic overhead Adds some threshold to avoid expensive cases, like: ``` #!python open('a', 'w').write(" \n" * 1000000) open('b', 'w').write(" \n" * 1000001) ``` The indent heuristic is O(N * 20) (N = 1000000) for the above case, and makes diff much slower. Before this patch (system git 2.14.2): ``` git diff --no-indent-heuristic a b 0.21s user 0.03s system 100% cpu 0.239 total git diff --indent-heuristic a b 0.77s user 0.02s system 99% cpu 0.785 total ``` After this patch (git 2fc74f41, with xdiffi.c patched): ``` # with the changed xdiffi.c git diff --indent-heuristic a b 0.16s user 0.01s system 90% cpu 0.188 total git diff --no-indent-heuristic a b 0.18s user 0.01s system 99% cpu 0.192 total ``` Now turning on indent-heuristic has no visible impact on performance. Differential Revision: https://phab.mercurial-scm.org/D2601

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