diffcore-rename: allow different missing_object_cb functions

estimate_similarity() was setting up a diff_populate_filespec_options
every time it was called, requiring the caller of estimate_similarity()
to pass in some data needed to set up this option.  Currently the needed
data consisted of a single variable (skip_unmodified), but we want to
also have the different estimate_similarity() callsites start using
different missing_object_cb functions as well.  Rather than also passing
that data in, just have the caller pass in the whole
diff_populate_filespec_options, and reduce the number of times we need to
set it up.

As a side note, this also drops the number of calls to
has_promisor_remote() dramatically.  If L is the number of basename
paths to compare, M is the number of inexact sources, and N is the
number of inexact destinations, then the number of calls to
has_promisor_remote() drops from L+M*N down to at most 2 -- one for each
of the sites that calls estimate_similarity().  has_promisor_remote() is
a very fast function so this almost certainly has no measurable
performance impact, but it seems cleaner to avoid calling that function
so many times.

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

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