rebase -p: fix quoting when calling `git merge`

It has been reported that strategy arguments are not passed to `git
merge` correctly when rebasing interactively, preserving merges.

The reason is that the strategy arguments are already quoted, and then
quoted again.

This fixes https://github.com/git-for-windows/git/issues/1321

Original-patch-by: Kim Gybels <kgybels@infogroep.be>
Also-reported-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -392,9 +392,12 @@ pick_one_preserving_merges () {
 			new_parents=${new_parents# $first_parent}
 			merge_args="--no-log --no-ff"
 			if ! do_with_author output eval \
-			'git merge ${gpg_sign_opt:+"$gpg_sign_opt"} \
-				$allow_rerere_autoupdate $merge_args \
-				$strategy_args -m "$msg_content" $new_parents'
+				git merge ${gpg_sign_opt:+$(git rev-parse \
+					--sq-quote "$gpg_sign_opt")} \
+				$allow_rerere_autoupdate "$merge_args" \
+				"$strategy_args" \
+				-m $(git rev-parse --sq-quote "$msg_content") \
+				"$new_parents"
 			then
 				printf "%s\n" "$msg_content" > "$GIT_DIR"/MERGE_MSG
 				die_with_patch $sha1 "$(eval_gettext "Error redoing merge \$sha1")"

t/t3418-rebase-continue.sh

@@ -74,6 +74,20 @@ test_expect_success 'rebase --continue remembers merge strategy and options' '
 	test -f funny.was.run
 '
 
+test_expect_success 'rebase passes merge strategy options correctly' '
+	rm -fr .git/rebase-* &&
+	git reset --hard commit-new-file-F3-on-topic-branch &&
+	test_commit theirs-to-merge &&
+	git reset --hard HEAD^ &&
+	test_commit some-commit &&
+	test_tick &&
+	git merge --no-ff theirs-to-merge &&
+	FAKE_LINES="1 edit 2 3" git rebase -i -f -p -m \
+		-s recursive --strategy-option=theirs HEAD~2 &&
+	test_commit force-change &&
+	git rebase --continue
+'
+
 test_expect_success 'setup rerere database' '
 	rm -fr .git/rebase-* &&
 	git reset --hard commit-new-file-F3-on-topic-branch &&