Merge branch 'rs/xdiff-lose-emit-func'

Simplifies the interface between the implementation of "blame" and
underlying xdiff engine, and removes a lot of unused or unnecessary code
from the latter.

By René Scharfe (6) and Ramsay Jones (1)
* rs/xdiff-lose-emit-func:
builtin/blame.c: Fix a "Using plain integer as NULL pointer" warning
xdiff: remove unused functions
xdiff: remove emit_func() and xdi_diff_hunks()
blame: factor out helper for calling xdi_diff()
blame: use hunk_func(), part 2
blame: use hunk_func(), part 1
xdiff: add hunk_func()