diffcore-rename: compute dir_rename_guess from dir_rename_counts

diffcore-rename: compute dir_rename_guess from dir_rename_counts

dir_rename_counts has a mapping of a mapping, in particular, it has
   old_dir => { new_dir => count }
We want a simple mapping of
   old_dir => new_dir
based on which new_dir had the highest count for a given old_dir.
Compute this and store it in dir_rename_guess.

This is the final piece of the puzzle needed to make our guesses at
which directory files have been moved to when basenames aren't unique.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:       12.775 s ±  0.062 s    12.596 s ±  0.061 s
    mega-renames:    188.754 s ±  0.284 s   130.465 s ±  0.259 s
    just-one-mega:     5.599 s ±  0.019 s     3.958 s ±  0.010 s

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>