diffcore-rename.c: simplify finding exact renames
The find_exact_renames function currently only uses the hash table for
grouping, i.e.:
1. add sources
2. add destinations
3. iterate all buckets, per bucket:
4. split sources from destinations
5. iterate destinations, per destination:
6. iterate sources to find best match
This can be simplified by utilizing the lookup functionality of the hash
table, i.e.:
1. add sources
2. iterate destinations, per destination:
3. lookup sources matching the current destination
4. iterate sources to find best match
This saves several iterations and file_similarity allocations for the
destinations.
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>