rebase: introduce the --rebase-merges option

Once upon a time, this here developer thought: wouldn't it be nice if,
say, Git for Windows' patches on top of core Git could be represented as
a thicket of branches, and be rebased on top of core Git in order to
maintain a cherry-pick'able set of patch series?

The original attempt to answer this was: git rebase --preserve-merges.

However, that experiment was never intended as an interactive option,
and it only piggy-backed on git rebase --interactive because that
command's implementation looked already very, very familiar: it was
designed by the same person who designed --preserve-merges: yours truly.

Some time later, some other developer (I am looking at you, Andreas!
;-)) decided that it would be a good idea to allow --preserve-merges to
be combined with --interactive (with caveats!) and the Git maintainer
(well, the interim Git maintainer during Junio's absence, that is)
agreed, and that is when the glamor of the --preserve-merges design
started to fall apart rather quickly and unglamorously.

The reason? In --preserve-merges mode, the parents of a merge commit (or
for that matter, of *any* commit) were not stated explicitly, but were
*implied* by the commit name passed to the `pick` command.

This made it impossible, for example, to reorder commits. Not to mention
to move commits between branches or, deity forbid, to split topic branches
into two.

Alas, these shortcomings also prevented that mode (whose original
purpose was to serve Git for Windows' needs, with the additional hope
that it may be useful to others, too) from serving Git for Windows'
needs.

Five years later, when it became really untenable to have one unwieldy,
big hodge-podge patch series of partly related, partly unrelated patches
in Git for Windows that was rebased onto core Git's tags from time to
time (earning the undeserved wrath of the developer of the ill-fated
git-remote-hg series that first obsoleted Git for Windows' competing
approach, only to be abandoned without maintainer later) was really
untenable, the "Git garden shears" were born [*1*/*2*]: a script,
piggy-backing on top of the interactive rebase, that would first
determine the branch topology of the patches to be rebased, create a
pseudo todo list for further editing, transform the result into a real
todo list (making heavy use of the `exec` command to "implement" the
missing todo list commands) and finally recreate the patch series on
top of the new base commit.

That was in 2013. And it took about three weeks to come up with the
design and implement it as an out-of-tree script. Needless to say, the
implementation needed quite a few years to stabilize, all the while the
design itself proved itself sound.

With this patch, the goodness of the Git garden shears comes to `git
rebase -i` itself. Passing the `--rebase-merges` option will generate
a todo list that can be understood readily, and where it is obvious
how to reorder commits. New branches can be introduced by inserting
`label` commands and calling `merge <label>`. And once this mode will
have become stable and universally accepted, we can deprecate the design
mistake that was `--preserve-merges`.

Link *1*:
https://github.com/msysgit/msysgit/blob/master/share/msysGit/shears.sh
Link *2*:
https://github.com/git-for-windows/build-extra/blob/master/shears.sh

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 179 insertions, 0 deletions

diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
new file mode 100755
index 0000000..5f0febb
--- /dev/null
+++ b/t/t3430-rebase-merges.sh
@@ -0,0 +1,179 @@
+#!/bin/sh
+#
+# Copyright (c) 2018 Johannes E. Schindelin
+#
+
+test_description='git rebase -i --rebase-merges
+
+This test runs git rebase "interactively", retaining the branch structure by
+recreating merge commits.
+
+Initial setup:
+
+    -- B --                   (first)
+   /       \
+ A - C - D - E - H            (master)
+       \       /
+         F - G                (second)
+'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-rebase.sh
+
+test_cmp_graph () {
+	cat >expect &&
+	git log --graph --boundary --format=%s "$@" >output &&
+	sed "s/ *$//" <output >output.trimmed &&
+	test_cmp expect output.trimmed
+}
+
+test_expect_success 'setup' '
+	write_script replace-editor.sh <<-\EOF &&
+	mv "$1" "$(git rev-parse --git-path ORIGINAL-TODO)"
+	cp script-from-scratch "$1"
+	EOF
+
+	test_commit A &&
+	git checkout -b first &&
+	test_commit B &&
+	git checkout master &&
+	test_commit C &&
+	test_commit D &&
+	git merge --no-commit B &&
+	test_tick &&
+	git commit -m E &&
+	git tag -m E E &&
+	git checkout -b second C &&
+	test_commit F &&
+	test_commit G &&
+	git checkout master &&
+	git merge --no-commit G &&
+	test_tick &&
+	git commit -m H &&
+	git tag -m H H
+'
+
+test_expect_success 'create completely different structure' '
+	cat >script-from-scratch <<-\EOF &&
+	label onto
+
+	# onebranch
+	pick G
+	pick D
+	label onebranch
+
+	# second
+	reset onto
+	pick B
+	label second
+
+	reset onto
+	merge -C H second
+	merge onebranch # Merge the topic branch '\''onebranch'\''
+	EOF
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_tick &&
+	git rebase -i -r A &&
+	test_cmp_graph <<-\EOF
+	*   Merge the topic branch '\''onebranch'\''
+	|\
+	| * D
+	| * G
+	* |   H
+	|\ \
+	| |/
+	|/|
+	| * B
+	|/
+	* A
+	EOF
+'
+
+test_expect_success 'generate correct todo list' '
+	cat >expect <<-\EOF &&
+	label onto
+
+	reset onto
+	pick d9df450 B
+	label E
+
+	reset onto
+	pick 5dee784 C
+	label branch-point
+	pick ca2c861 F
+	pick 088b00a G
+	label H
+
+	reset branch-point # C
+	pick 12bd07b D
+	merge -C 2051b56 E # E
+	merge -C 233d48a H # H
+
+	EOF
+
+	grep -v "^#" <.git/ORIGINAL-TODO >output &&
+	test_cmp expect output
+'
+
+test_expect_success '`reset` refuses to overwrite untracked files' '
+	git checkout -b refuse-to-reset &&
+	test_commit dont-overwrite-untracked &&
+	git checkout @{-1} &&
+	: >dont-overwrite-untracked.t &&
+	echo "reset refs/tags/dont-overwrite-untracked" >script-from-scratch &&
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_must_fail git rebase -r HEAD &&
+	git rebase --abort
+'
+
+test_expect_success 'failed `merge` writes patch (may be rescheduled, too)' '
+	test_when_finished "test_might_fail git rebase --abort" &&
+	git checkout -b conflicting-merge A &&
+
+	: fail because of conflicting untracked file &&
+	>G.t &&
+	echo "merge -C H G" >script-from-scratch &&
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_tick &&
+	test_must_fail git rebase -ir HEAD &&
+	grep "^merge -C .* G$" .git/rebase-merge/done &&
+	grep "^merge -C .* G$" .git/rebase-merge/git-rebase-todo &&
+	test_path_is_file .git/rebase-merge/patch &&
+
+	: fail because of merge conflict &&
+	rm G.t .git/rebase-merge/patch &&
+	git reset --hard &&
+	test_commit conflicting-G G.t not-G conflicting-G &&
+	test_must_fail git rebase --continue &&
+	! grep "^merge -C .* G$" .git/rebase-merge/git-rebase-todo &&
+	test_path_is_file .git/rebase-merge/patch
+'
+
+test_expect_success 'with a branch tip that was cherry-picked already' '
+	git checkout -b already-upstream master &&
+	base="$(git rev-parse --verify HEAD)" &&
+
+	test_commit A1 &&
+	test_commit A2 &&
+	git reset --hard $base &&
+	test_commit B1 &&
+	test_tick &&
+	git merge -m "Merge branch A" A2 &&
+
+	git checkout -b upstream-with-a2 $base &&
+	test_tick &&
+	git cherry-pick A2 &&
+
+	git checkout already-upstream &&
+	test_tick &&
+	git rebase -i -r upstream-with-a2 &&
+	test_cmp_graph upstream-with-a2.. <<-\EOF
+	*   Merge branch A
+	|\
+	| * A1
+	* | B1
+	|/
+	o A2
+	EOF
+'
+
+test_done