merge-ort: precompute whether directory rename detection is needed

The point of directory rename detection is that if one side of history
renames a directory, and the other side adds new files under the old
directory, then the merge can move those new files into the new
directory.  This leads to the following important observation:

  * If the other side does not add any new files under the old
    directory, we do not need to detect any renames for that directory.

Similarly, directory rename detection had an important requirement:

  * If a directory still exists on one side of history, it has not been
    renamed on that side of history.  (See section 4 of t6423 or
    Documentation/technical/directory-rename-detection.txt for more
    details).

Using these two bits of information, we note that directory rename
detection is only needed in cases where (1) directories exist in the
merge base and on one side of history (i.e. dirmask == 3 or dirmask ==
5), and (2) where there is some new file added to that directory on the
side where it still exists (thus where the file has filemask == 2 or
filemask == 4, respectively).  This has to be done in two steps, because
we have the dirmask when we are first considering the directory, and
won't get the filemasks for the files within it until we recurse into
that directory.  So, we save
  dir_rename_mask = dirmask - 1
when we hit a directory that is missing on one side, and then later look
for cases of
  filemask == dir_rename_mask

One final note is that as soon as we hit a directory that needs
directory rename detection, we will need to detect renames in all
subdirectories of that directory as well due to the "majority rules"
decision when files are renamed into different directory hierarchies.
We arbitrarily use the special value of 0x07 to record when we've hit
such a directory.

The combination of all the above mean that we introduce a variable
named dir_rename_mask (couldn't think of a better name) which has one
of the following values as we traverse into a directory:
   * 0x00: directory rename detection not needed
   * 0x02 or 0x04: directory rename detection only needed if files added
   * 0x07: directory rename detection definitely needed

We then pass this value through to add_pairs() so that it can mark
location_relevant as true only when dir_rename_mask is 0x07.

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

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