diffcore-rename: use a mem_pool for exact rename detection's hashmap

diffcore-rename: use a mem_pool for exact rename detection's hashmap

Exact rename detection, via insert_file_table(), uses a hashmap to store
files by oid.  Use a mem_pool for the hashmap entries so these can all be
allocated and deallocated together.

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:      204.2  ms ±  3.0  ms   202.5  ms ±  3.2  ms
    mega-renames:      1.076 s ±  0.015 s     1.072 s ±  0.012 s
    just-one-mega:   364.1  ms ±  7.0  ms   357.3  ms ±  3.9  ms

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>