merge-ort: clarify the interning of strings in opt->priv->path

merge-ort: clarify the interning of strings in opt->priv->path

Because merge-ort is dealing with potentially all the pathnames in the
repository, it sometimes needs to do an awful lot of string comparisons.
Because of this, struct merge_options_internal's path member was
envisioned from the beginning to contain an interned value for every
path in order to allow us to compare strings via pointer comparison
instead of using strcmp.  See
  * 5b59c3db059d (merge-ort: setup basic internal data structures,
                  2020-12-13)
  * f591c4724615 (merge-ort: copy and adapt merge_3way() from
                  merge-recursive.c, 2021-01-01)
for some of the early comments.

However, the original comment was slightly misleading when it switched
from mentioning paths to only mentioning directories.  Fix that, and
while at it also point to an example in the code which applies the extra
needed care to permit the pointer comparison optimization.

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