Merge branch 'sb/histogram-less-memory'

"git diff --histogram" had a bad memory usage pattern, which has
been rearranged to reduce the peak usage.

* sb/histogram-less-memory:
  xdiff/histogram: remove tail recursion
  xdiff/xhistogram: move index allocation into find_lcs
  xdiff/xhistogram: factor out memory cleanup into free_index()
  xdiff/xhistogram: pass arguments directly to fall_back_to_classic_diff