"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