Merge branch 'en/rebase-consistency'

"git rebase" behaved slightly differently depending on which one of
the three backends gets used; this has been documented and an
effort to make them more uniform has begun.

* en/rebase-consistency:
  git-rebase: make --allow-empty-message the default
  t3401: add directory rename testcases for rebase and am
  git-rebase.txt: document behavioral differences between modes
  directory-rename-detection.txt: technical docs on abilities and limitations
  git-rebase.txt: address confusion between --no-ff vs --force-rebase
  git-rebase: error out when incompatible options passed
  t3422: new testcases for checking when incompatible options passed
  git-rebase.sh: update help messages a bit
  git-rebase.txt: document incompatible options

1 files changed, 115 insertions, 0 deletions

diff --git a/Documentation/technical/directory-rename-detection.txt b/Documentation/technical/directory-rename-detection.txt
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+Directory rename detection
+==========================
+
+Rename detection logic in diffcore-rename that checks for renames of
+individual files is aggregated and analyzed in 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.
+
+  * 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/t6043-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/t6043-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) Avoid directory-rename-detection for a path, if that path is the
+     source of a rename on either side of a merge.
+
+  c) Only apply implicit directory renames to directories if the other side
+     of history is the one doing the renaming.
+
+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.  Further, to implement this, directory rename
+    detection logic would need to move from merge-recursive to
+    diffcore-rename.
+
+  * 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.