[PATCH v2 08/15] git-rebase.txt: add more details about behavioral differences of backends

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 Documentation/git-rebase.txt            | 102 +++++++++++++++++++++---
 t/t3433-rebase-options-compatibility.sh |   5 +-
 2 files changed, 94 insertions(+), 13 deletions(-)

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index ff32ca1080..f1ace07c38 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -409,13 +409,10 @@ your branch contains commits which were dropped, this option can be used
 with `--keep-base` in order to drop those commits from your branch.
 
 --ignore-whitespace::
-	Behaves differently depending on which backend is selected.
-+
-'am' backend: When applying a patch, ignore changes in whitespace in
-context lines if necessary.
-+
-'interactive' backend: Treat lines with only whitespace changes as
-unchanged for the sake of a three-way merge.
+	Ignore whitespace-only changes in the commits being rebased,
+	which may avoid "unnecessary" conflicts.  (Both backends
+	currently have differing edgecase bugs with this option; see
+	BEHAVIORAL DIFFERENCES.)
 
 --whitespace=<option>::
 	This flag is passed to the 'git apply' program
@@ -609,9 +606,94 @@ There are some subtle differences how the backends behave.
 Directory rename detection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Directory rename heuristics are enabled in the merge and interactive
-backends.  Due to the lack of accurate tree information, directory
-rename detection is disabled in the am backend.
+Due to the lack of accurate tree information (arising from
+constructing fake ancestors with the limited information available in
+patches), directory rename detection is disabled in the am backend.
+Disabled directory rename detection means that if one side of history
+renames a directory and the other adds new files to the old directory,
+then the new files will be left behind in the old directory without
+any warning at the time of rebasing that you may want to move these
+files into the new directory.
+
+Directory rename detection works with the merge and interactive
+backends to provide you warnings in such cases.
+
+Context
+~~~~~~~
+
+The am backend works by creating a sequence of patches (by calling
+`format-patch` internally), and then applying the patches in sequence
+(calling `am` internally).  Patches are composed of multiple hunks,
+each with line numbers, a context region, and the actual changes.  The
+line numbers have to be taken with some fuzz, since the other side
+will likely have inserted or deleted lines earlier in the file.  The
+context region is meant to help find how to adjust the line numbers in
+order to apply the changes to the right lines.  However, if multiple
+areas of the code have the same surrounding lines of context, the
+wrong one can be picked.  There are real-world cases where this has
+caused commits to be reapplied incorrectly with no conflicts reported.
+Setting diff.context to a larger value may prevent such types of
+problems, but increases the chance of spurious conflicts (since it
+will require more lines of matching context to apply).
+
+The interactive backend works with a full copy of each relevant file,
+insulating it from these types of problems.
+
+Labelling of conflicts markers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When there are content conflicts, the merge machinery tries to
+annotate each side's conflict markers with the commits where the
+content came from.  Since the am backend drops the original
+information about the rebased commits and their parents (and instead
+generates new fake commits based off limited information in the
+generated patches), those commits cannot be identified; instead it has
+to fall back to a commit summary.  Also, when merge.conflictStyle is
+set to diff3, the am backend will use "constructed merge base" to
+label the content from the merge base, and thus provide no information
+about the merge base commit whatsoever.
+
+The interactive backend works with the full commits on both sides of
+history and thus has no such limitations.
+
+--ignore-whitespace
+~~~~~~~~~~~~~~~~~~~
+
+The --ignore-whitespace option is supposed to ignore whitespace-only
+changes if it allows the code to merge cleanly.  Unfortunately, the
+different backends implement this differently, and both have different
+edge case bugs.
++
+'am' backend: When applying a patch, ignore changes in whitespace in
+context lines if necessary.  (Which implies that if the whitespace
+change was not in the context lines but on a line with a real change,
+then the rebase will still fail with "unnecessary" content conflicts.)
++
+'interactive' backend: Treat lines with only whitespace changes as
+unchanged for the sake of a three-way merge.  This means that if one
+side made no changes and the commits being rebased had whitespace-only
+changes, those whitespaces fixups will be discarded despite the fact
+that they present no content conflict.
+
+Miscellaneous differences
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are a few more behavioral differences that most folks would
+probably consider inconsequential but which are mentioned for
+completeness:
+
+* Reflog: The two backends will use different wording when describing
+  the changes made in the reflog, though both will make use of the
+  word "rebase".
+
+* Progress, informational, and error messages: The two backends
+  provide slightly different progress and informational messages.
+  Also, the am backend writes error messages (such as "Your files
+  would be overwritten...") to stdout, while the interactive backend
+  writes them to stderr.
+
+* State directories: The two backends keep their state in different
+  directories under .git/
 
 include::merge-strategies.txt[]
 
diff --git a/t/t3433-rebase-options-compatibility.sh b/t/t3433-rebase-options-compatibility.sh
index 5166f158dd..bd4d2d2f63 100755
--- a/t/t3433-rebase-options-compatibility.sh
+++ b/t/t3433-rebase-options-compatibility.sh
@@ -10,9 +10,8 @@ test_description='tests to ensure compatibility between am and interactive backe
 GIT_AUTHOR_DATE="1999-04-02T08:03:20+05:30"
 export GIT_AUTHOR_DATE
 
-# This is a special case in which both am and interactive backends
-# provide the same output. It was done intentionally because
-# both the backends fall short of optimal behaviour.
+# This is a common case in which both am and interactive backends
+# provide the same output with --ignore-whitespace.
 test_expect_success 'setup' '
 	git checkout -b topic &&
 	q_to_tab >file <<-\EOF &&
-- 
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