merge-ort: add data structures for allowable trivial directory resolves

As noted a few commits ago, we can resolve individual files early if all
three sides of the merge have a file at the path and two of the three
sides match.  We would really like to do the same thing with
directories, because being able to do a trivial directory resolve means
we don't have to recurse into the directory, potentially saving us a
huge amount of time in both collect_merge_info() and process_entries().
Unfortunately, resolving directories early would mean missing any
renames whose source or destination is underneath that directory.

If we somehow knew there weren't any renames under the directory in
question, then we could resolve it early.  Sadly, it is impossible to
determine whether there are renames under the directory in question
without recursing into it, and this has traditionally kept us from ever
implementing such an optimization.

In commit f89b4f2bee ("merge-ort: skip rename detection entirely if
possible", 2021-03-11), we added an additional reason that rename
detection could be skipped entirely -- namely, if no *relevant* sources
were present.  Without completing collect_merge_info_callback(), we do
not yet know if there are no relevant sources.  However, we do know that
if the current directory on one side matches the merge base, then every
source file within that directory will not be RELEVANT_CONTENT, and a
few simple checks can often let us rule out RELEVANT_LOCATION as well.
This suggests we can just defer recursing into such directories until
the end of collect_merge_info.

Since the deferred directories are known to not add any relevant sources
due to the above properties, then if there are no relevant sources after
we've traversed all paths other than the deferred ones, then we know
there are not any relevant sources.  Under those conditions, rename
detection is unnecessary, and that means we can resolve the deferred
directories without recursing into them.

Note that the logic for skipping rename detection was also modified
further in commit 76e253793c ("merge-ort, diffcore-rename: employ cached
renames when possible", 2021-01-30); in particular rename detection can
be skipped if we already have cached renames for each relevant source.
We can take advantage of this information as well with our deferral of
recursing into directories where one side matches the merge base.

Add some data structures that we will use to do these deferrals, with
some lengthy comments explaining their purpose.

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

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