diffcore-rename: take advantage of "majority rules" to skip more renames

diffcore-rename: take advantage of "majority rules" to skip more renames

In directory rename detection (when a directory is removed on one side
of history and the other side adds new files to that directory), we work
to find where the greatest number of files within that directory were
renamed to so that the new files can be moved with the majority of the
files.

Naively, we can just do this by detecting renames for *all* files within
the removed/renamed directory, looking at all the destination
directories where files within that directory were moved, and if there
is more than one such directory then taking the one with the greatest
number of files as the directory where the old directory was renamed to.

However, sometimes there are enough renames from exact rename detection
or basename-guided rename detection that we have enough information to
determine the majority winner already.  Add a function meant to compute
whether particular renames are still needed based on this majority rules
check.  The next several commits will then add the necessary
infrastructure to get the information we need to compute which
additional rename sources we can skip.

An important side note for future further optimization:

There is a possible improvement to this optimization that I have not yet
attempted and will not be included in this series of patches: we could
first check whether exact renames provide enough information for us to
determine directory renames, and avoid doing basename-guided rename
detection on some or all of the RELEVANT_LOCATION files within those
directories.  In effect, this variant would mean doing the
handle_early_known_dir_renames() both after exact rename detection and
again after basename-guided rename detection, though it would also mean
decrementing the number of "unknown" renames for each rename we found
from basename-guided rename detection.  Adding this additional check for
skippable renames right after exact rename detection might turn out to
be valuable, especially for partial clones where it might allow us to
download certain source files entirely.  However, this particular
optimization was actually the last one I did in original implementation
order, and by the time I implemented this idea, every testcase I had was
sufficiently fast that further optimization was unwarranted.  If future
testcases arise that tax rename detection more heavily (or perhaps
partial clones can benefit from avoiding loading more objects), it may
be worth implementing this more involved variant.

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