rebase (interactive-backend): make --keep-empty the default

Different rebase backends have different treatment for commits which
start empty (i.e. have no changes relative to their parent), and the
--keep-empty option was added at some point to allow adjusting behavior.
The handling of commits which start empty is actually quite similar to
commit b00bf1c9a8dd (git-rebase: make --allow-empty-message the default,
2018-06-27), which pointed out that the behavior for various backends is
often more happenstance than design.  The specific change made in that
commit is actually quite relevant as well and much of the logic there
directly applies here.

It makes a lot of sense in 'git commit' to error out on the creation of
empty commits, unless an override flag is provided.  However, once
someone determines that there is a rare case that merits using the
manual override to create such a commit, it is somewhere between
annoying and harmful to have to take extra steps to keep such
intentional commits around.  Granted, empty commits are quite rare,
which is why handling of them doesn't get considered much and folks tend
to defer to existing (accidental) behavior and assume there was a reason
for it, leading them to just add flags (--keep-empty in this case) that
allow them to override the bad defaults.  Fix the interactive backend so
that --keep-empty is the default, much like we did with
--allow-empty-message.  The am backend should also be fixed to have
--keep-empty semantics for commits that start empty, but that is not
included in this patch other than a testcase documenting the failure.

Note that there was one test in t3421 which appears to have been written
expecting --keep-empty to not be the default as correct behavior.  This
test was introduced in commit 00b8be5a4d38 ("add tests for rebasing of
empty commits", 2013-06-06), which was part of a series focusing on
rebase topology and which had an interesting original cover letter at
https://lore.kernel.org/git/1347949878-12578-1-git-send-email-martinvonz@gmail.com/
which noted
    Your input especially appreciated on whether you agree with the
    intent of the test cases.
and then went into a long example about how one of the many tests added
had several questions about whether it was correct.  As such, I believe
most the tests in that series were about testing rebase topology with as
many different flags as possible and were not trying to state in general
how those flags should behave otherwise.

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

1 files changed, 13 insertions, 11 deletions

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index c83be7ffc2..1d19542d79 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -259,10 +259,13 @@ See also INCOMPATIBLE OPTIONS below.
 	unchanged as a result.
 
 --keep-empty::
-	Keep the commits that do not change anything from its
-	parents in the result.
+	No-op.  Rebasing commits that started empty (had no change
+	relative to their parent) used to fail and this option would
+	override that behavior, allowing commits with empty changes to
+	be rebased.  Now commits with no changes do not cause rebasing
+	to halt.
 +
-See also INCOMPATIBLE OPTIONS below.
+See also BEHAVIORAL DIFFERENCES and INCOMPATIBLE OPTIONS below.
 
 --allow-empty-message::
 	No-op.  Rebasing commits with an empty message used to fail
@@ -577,15 +580,14 @@ There are some subtle differences how the backends behave.
 Empty commits
 ~~~~~~~~~~~~~
 
-The am backend drops any "empty" commits, regardless of whether the
-commit started empty (had no changes relative to its parent to
-start with) or ended empty (all changes were already applied
-upstream in other commits).
+The am backend unfortunately drops intentionally empty commits, i.e.
+commits that started empty, though these are rare in practice.  It
+also drops commits that become empty and has no option for controlling
+this behavior.
 
-The interactive backend drops commits by default that
-started empty and halts if it hits a commit that ended up empty.
-The `--keep-empty` option exists for the interactive backend to allow
-it to keep commits that started empty.
+The interactive backend keeps intentionally empty commits.
+Unfortunately, it always halts whenever it runs across a commit that
+becomes empty, even when the rebase is not explicitly interactive.
 
 Directory rename detection
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