sequencer: do not squash 'reword' commits when we hit conflicts

Ever since commit 18633e1a ("rebase -i: use the rebase--helper builtin",
2017-02-09), when a commit marked as 'reword' in an interactive rebase
has conflicts and fails to apply, when the rebase is resumed that commit
will be squashed into its parent with its commit message taken.

The issue can be understood better by looking at commit 56dc3ab0
("sequencer (rebase -i): implement the 'edit' command", 2017-01-02), which
introduced error_with_patch() for the edit command.  For the edit command,
it needs to stop the rebase whether or not the patch applies cleanly.  If
the patch does apply cleanly, then when it resumes it knows it needs to
amend all changes into the previous commit.  If it does not apply cleanly,
then the changes should not be amended.  Thus, it passes !res (success of
applying the 'edit' commit) to error_with_patch() for the to_amend flag.

The problematic line of code actually came from commit 04efc8b5
("sequencer (rebase -i): implement the 'reword' command", 2017-01-02).
Note that to get to this point in the code:
  * !!res (i.e. patch application failed)
  * item->command < TODO_SQUASH
  * item->command != TODO_EDIT
  * !is_fixup(item->command) [i.e. not squash or fixup]
So that means this can only be a failed patch application that is either a
pick, revert, or reword.  We only need to amend HEAD when rewording the
root commit or a commit that has been fast-forwarded, for any of the other
cases we want a new commit, so we should not set the to_amend flag.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Original-patch-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3214,10 +3214,27 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 					intend_to_amend();
 				return error_failed_squash(item->commit, opts,
 					item->arg_len, item->arg);
-			} else if (res && is_rebase_i(opts) && item->commit)
+			} else if (res && is_rebase_i(opts) && item->commit) {
+				int to_amend = 0;
+				struct object_id oid;
+
+				/*
+				 * If we are rewording and have either
+				 * fast-forwarded already, or are about to
+				 * create a new root commit, we want to amend,
+				 * otherwise we do not.
+				 */
+				if (item->command == TODO_REWORD &&
+				    !get_oid("HEAD", &oid) &&
+				    (!oidcmp(&item->commit->object.oid, &oid) ||
+				     (opts->have_squash_onto &&
+				      !oidcmp(&opts->squash_onto, &oid))))
+					to_amend = 1;
+
 				return res | error_with_patch(item->commit,
-					item->arg, item->arg_len, opts, res,
-					item->command == TODO_REWORD);
+						item->arg, item->arg_len, opts,
+						res, to_amend);
+			}
 		} else if (item->command == TODO_EXEC) {
 			char *end_of_arg = (char *)(item->arg + item->arg_len);
 			int saved = *end_of_arg;

t/t3404-rebase-interactive.sh

@@ -981,7 +981,35 @@ test_expect_success 'rebase -i --root reword root commit' '
 	test -z "$(git show -s --format=%p HEAD^)"
 '
+test_expect_success 'rebase -i --root when root has untracked file confilct' '
+	test_when_finished "reset_rebase" &&
+	git checkout -b failing-root-pick A &&
+	echo x >file2 &&
+	git rm file1 &&
+	git commit -m "remove file 1 add file 2" &&
+	echo z >file1 &&
+	set_fake_editor &&
+	test_must_fail env FAKE_LINES="1 2" git rebase -i --root &&
+	rm file1 &&
+	git rebase --continue &&
+	test "$(git log -1 --format=%B)" = "remove file 1 add file 2" &&
+	test "$(git rev-list --count HEAD)" = 2
+'
+
+test_expect_success 'rebase -i --root reword root when root has untracked file conflict' '
+	test_when_finished "reset_rebase" &&
+	echo z>file1 &&
+	set_fake_editor &&
+	test_must_fail env FAKE_LINES="reword 1 2" \
+		FAKE_COMMIT_MESSAGE="Modified A" git rebase -i --root &&
+	rm file1 &&
+	FAKE_COMMIT_MESSAGE="Reworded A" git rebase --continue &&
+	test "$(git log -1 --format=%B HEAD^)" = "Reworded A" &&
+	test "$(git rev-list --count HEAD)" = 2
+'
+
 test_expect_success C_LOCALE_OUTPUT 'rebase --edit-todo does not work on non-interactive rebase' '
+	git checkout reword-root-branch &&
 	git reset --hard &&
 	git checkout conflict-branch &&
 	set_fake_editor &&

t/t3423-rebase-reword.sh

+#!/bin/sh
+
+test_description='git rebase interactive with rewording'
+
+. ./test-lib.sh
+
+. "$TEST_DIRECTORY"/lib-rebase.sh
+
+test_expect_success 'setup' '
+	test_commit master file-1 test &&
+
+	git checkout -b stuff &&
+
+	test_commit feature_a file-2 aaa &&
+	test_commit feature_b file-2 ddd
+'
+
+test_expect_success 'reword without issues functions as intended' '
+	test_when_finished "reset_rebase" &&
+
+	git checkout stuff^0 &&
+
+	set_fake_editor &&
+	FAKE_LINES="pick 1 reword 2" FAKE_COMMIT_MESSAGE="feature_b_reworded" \
+		git rebase -i -v master &&
+
+	test "$(git log -1 --format=%B)" = "feature_b_reworded" &&
+	test $(git rev-list --count HEAD) = 3
+'
+
+test_expect_success 'reword after a conflict preserves commit' '
+	test_when_finished "reset_rebase" &&
+
+	git checkout stuff^0 &&
+
+	set_fake_editor &&
+	test_must_fail env FAKE_LINES="reword 2" \
+		git rebase -i -v master &&
+
+	git checkout --theirs file-2 &&
+	git add file-2 &&
+	FAKE_COMMIT_MESSAGE="feature_b_reworded" git rebase --continue &&
+
+	test "$(git log -1 --format=%B)" = "feature_b_reworded" &&
+	test $(git rev-list --count HEAD) = 2
+'
+
+test_done