rebase -i --root: let the sequencer handle even the initial part

In this developer's earlier attempt to accelerate interactive rebases by
converting large parts from Unix shell script into portable, performant
C, the --root handling was specifically excluded (to simplify the task a
little bit; it still took over a year to get that reduced set of patches
into Git proper).

This patch ties up that loose end: now only --preserve-merges uses the
slow Unix shell script implementation to perform the interactive rebase.

As the rebase--helper reports progress to stderr (unlike the scripted
interactive rebase, which reports it to stdout, of all places), we have
to adjust a couple of tests that did not expect that for `git rebase -i
--root`.

This patch fixes -- at long last! -- the really old bug reported in
6a6bc5bdc4 (add tests for rebasing root, 2013-06-06) that rebasing with
--root *always* rewrote the root commit, even if there were no changes.

The bug still persists in --preserve-merges mode, of course, but that
mode will be deprecated as soon as the new --rebase-merges mode
stabilizes, anyway.

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

git-rebase--interactive.sh

@@ -894,6 +894,8 @@ init_revisions_and_shortrevisions () {
 	else
 		revisions=$onto...$orig_head
 		shortrevisions=$shorthead
+		test -z "$squash_onto" ||
+		echo "$squash_onto" >"$state_dir"/squash-onto
 	fi
 }
 
@@ -948,7 +950,7 @@ EOF
 	die "Could not skip unnecessary pick commands"
 
 	checkout_onto
-	if test -z "$rebase_root" && test ! -d "$rewritten"
+	if test ! -d "$rewritten"
 	then
 		require_clean_work_tree "rebase"
 		exec git rebase--helper ${force_rebase:+--no-ff} $allow_empty_message \

t/t3404-rebase-interactive.sh

@@ -1204,10 +1204,6 @@ test_expect_success 'drop' '
 	test A = $(git cat-file commit HEAD^^ | sed -ne \$p)
 '
 
-cat >expect <<EOF
-Successfully rebased and updated refs/heads/missing-commit.
-EOF
-
 test_expect_success 'rebase -i respects rebase.missingCommitsCheck = ignore' '
 	test_config rebase.missingCommitsCheck ignore &&
 	rebase_setup_and_clean missing-commit &&
@@ -1215,7 +1211,9 @@ test_expect_success 'rebase -i respects rebase.missingCommitsCheck = ignore' '
 	FAKE_LINES="1 2 3 4" \
 		git rebase -i --root 2>actual &&
 	test D = $(git cat-file commit HEAD | sed -ne \$p) &&
-	test_i18ncmp expect actual
+	test_i18ngrep \
+		"Successfully rebased and updated refs/heads/missing-commit" \
+		actual
 '
 
 cat >expect <<EOF
@@ -1227,15 +1225,24 @@ To avoid this message, use "drop" to explicitly remove a commit.
 Use 'git config rebase.missingCommitsCheck' to change the level of warnings.
 The possible behaviours are: ignore, warn, error.
 
+Rebasing (1/4)
+Rebasing (2/4)
+Rebasing (3/4)
+Rebasing (4/4)
 Successfully rebased and updated refs/heads/missing-commit.
 EOF
 
+cr_to_nl () {
+	tr '\015' '\012'
+}
+
 test_expect_success 'rebase -i respects rebase.missingCommitsCheck = warn' '
 	test_config rebase.missingCommitsCheck warn &&
 	rebase_setup_and_clean missing-commit &&
 	set_fake_editor &&
 	FAKE_LINES="1 2 3 4" \
-		git rebase -i --root 2>actual &&
+		git rebase -i --root 2>actual.2 &&
+	cr_to_nl <actual.2 >actual &&
 	test_i18ncmp expect actual &&
 	test D = $(git cat-file commit HEAD | sed -ne \$p)
 '

t/t3421-rebase-topology-linear.sh

@@ -328,9 +328,9 @@ test_run_rebase () {
 		test_cmp_rev c HEAD
 	"
 }
-test_run_rebase failure ''
-test_run_rebase failure -m
-test_run_rebase failure -i
+test_run_rebase success ''
+test_run_rebase success -m
+test_run_rebase success -i
 test_run_rebase failure -p
 
 test_run_rebase () {