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+Directory rename detection
+==========================
+
+Rename detection logic in diffcore-rename that checks for renames of
+individual files is also aggregated there and then analyzed in either
+merge-ort or merge-recursive for cases where combinations of renames
+indicate that a full directory has been renamed.
+
+Scope of abilities
+------------------
+
+It is perhaps easiest to start with an example:
+
+ * When all of x/a, x/b and x/c have moved to z/a, z/b and z/c, it is
+ likely that x/d added in the meantime would also want to move to z/d by
+ taking the hint that the entire directory 'x' moved to 'z'.
+
+More interesting possibilities exist, though, such as:
+
+ * one side of history renames x -> z, and the other renames some file to
+ x/e, causing the need for the merge to do a transitive rename so that
+ the rename ends up at z/e.
+
+ * one side of history renames x -> z, but also renames all files within x.
+ For example, x/a -> z/alpha, x/b -> z/bravo, etc.
+
+ * both 'x' and 'y' being merged into a single directory 'z', with a
+ directory rename being detected for both x->z and y->z.
+
+ * not all files in a directory being renamed to the same location;
+ i.e. perhaps most the files in 'x' are now found under 'z', but a few
+ are found under 'w'.
+
+ * a directory being renamed, which also contained a subdirectory that was
+ renamed to some entirely different location. (And perhaps the inner
+ directory itself contained inner directories that were renamed to yet
+ other locations).
+
+ * combinations of the above; see t/t6423-merge-rename-directories.sh for
+ various interesting cases.
+
+Limitations -- applicability of directory renames
+-------------------------------------------------
+
+In order to prevent edge and corner cases resulting in either conflicts
+that cannot be represented in the index or which might be too complex for
+users to try to understand and resolve, a couple basic rules limit when
+directory rename detection applies:
+
+ 1) If a given directory still exists on both sides of a merge, we do
+ not consider it to have been renamed.
+
+ 2) If a subset of to-be-renamed files have a file or directory in the
+ way (or would be in the way of each other), "turn off" the directory
+ rename for those specific sub-paths and report the conflict to the
+ user.
+
+ 3) If the other side of history did a directory rename to a path that
+ your side of history renamed away, then ignore that particular
+ rename from the other side of history for any implicit directory
+ renames (but warn the user).
+
+Limitations -- detailed rules and testcases
+-------------------------------------------
+
+t/t6423-merge-rename-directories.sh contains extensive tests and commentary
+which generate and explore the rules listed above. It also lists a few
+additional rules:
+
+ a) If renames split a directory into two or more others, the directory
+ with the most renames, "wins".
+
+ b) Only apply implicit directory renames to directories if the other side
+ of history is the one doing the renaming.
+
+ c) Do not perform directory rename detection for directories which had no
+ new paths added to them.
+
+Limitations -- support in different commands
+--------------------------------------------
+
+Directory rename detection is supported by 'merge' and 'cherry-pick'.
+Other git commands which users might be surprised to see limited or no
+directory rename detection support in:
+
+ * diff
+
+ Folks have requested in the past that `git diff` detect directory
+ renames and somehow simplify its output. It is not clear whether this
+ would be desirable or how the output should be simplified, so this was
+ simply not implemented. Also, while diffcore-rename has most of the
+ logic for detecting directory renames, some of the logic is still found
+ within merge-ort and merge-recursive. Fully supporting directory
+ rename detection in diffs would require copying or moving the remaining
+ bits of logic to the diff machinery.
+
+ * am
+
+ git-am tries to avoid a full three way merge, instead calling
+ git-apply. That prevents us from detecting renames at all, which may
+ defeat the directory rename detection. There is a fallback, though; if
+ the initial git-apply fails and the user has specified the -3 option,
+ git-am will fall back to a three way merge. However, git-am lacks the
+ necessary information to do a "real" three way merge. Instead, it has
+ to use build_fake_ancestor() to get a merge base that is missing files
+ whose rename may have been important to detect for directory rename
+ detection to function.
+
+ * rebase
+
+ Since am-based rebases work by first generating a bunch of patches
+ (which no longer record what the original commits were and thus don't
+ have the necessary info from which we can find a real merge-base), and
+ then calling git-am, this implies that am-based rebases will not always
+ successfully detect directory renames either (see the 'am' section
+ above). merged-based rebases (rebase -m) and cherry-pick-based rebases
+ (rebase -i) are not affected by this shortcoming, and fully support
+ directory rename detection.