diffcore-rename: reduce jumpiness in progress counters

diffcore-rename: reduce jumpiness in progress counters

Inexact rename detection works by comparing all sources to all
destinations, computing similarities, and then finding the best matches
among those that are sufficiently similar.

However, it is preceded by exact rename detection that works by
checking if there are files with identical hashes.  If exact renames are
found, we can exclude some files from inexact rename detection.

The inexact rename detection loops over the full set of files, but
immediately skips those for which rename_dst[i].is_rename is true and
thus doesn't compare any sources to that destination.  As such, these
paths shouldn't be included in the progress counter.

For the eagle eyed, this change hints at an actual optimization -- the
first one I presented at Git Merge 2020.  I'll be submitting that
optimization later, once the basic merge-ort algorithm has merged.

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