diffcore-rename: refactor "too many candidates" logic

diffcore-rename: refactor "too many candidates" logic

Move the logic to a separate function, to be enhanced by later patches in
the series.

While at it, swap the condition used in the if statement from "if it is
too big then do this" to "if it would fit then do this".

Signed-off-by: Junio C Hamano <gitster@pobox.com>
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 Rebased to 'master' as the logic to use the result of this logic was
 updated recently, together with the addition of eye-candy.