rebase -i: handle core.commentChar=auto

When 84c9dc2 (commit: allow core.commentChar=auto for character auto
selection, 2014-05-17) extended the core.commentChar functionality to
allow for the value 'auto', it forgot that rebase -i was already taught to
handle core.commentChar, and in turn forgot to let rebase -i handle that
new value gracefully.

Reported by Taufiq Hoven.

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

git-rebase--interactive.sh

@@ -93,8 +93,17 @@ eval '
 GIT_CHERRY_PICK_HELP="$resolvemsg"
 export GIT_CHERRY_PICK_HELP
 
-comment_char=$(git config --get core.commentchar 2>/dev/null | cut -c1)
-: ${comment_char:=#}
+comment_char=$(git config --get core.commentchar 2>/dev/null)
+case "$comment_char" in
+'' | auto)
+	comment_char="#"
+	;;
+?)
+	;;
+*)
+	comment_char=$(echo "$comment_char" | cut -c1)
+	;;
+esac
 
 warn () {
 	printf '%s\n' "$*" >&2

t/t3404-rebase-interactive.sh

@@ -983,7 +983,7 @@ test_expect_success 'rebase -i respects core.commentchar' '
 	test B = $(git cat-file commit HEAD^ | sed -ne \$p)
 '
 
-test_expect_failure 'rebase -i respects core.commentchar=auto' '
+test_expect_success 'rebase -i respects core.commentchar=auto' '
 	test_config core.commentchar auto &&
 	write_script copy-edit-script.sh <<-\EOF &&
 	cp "$1" edit-script