sequencer: reencode to utf-8 before arrange rebase's todo list

sequencer: reencode to utf-8 before arrange rebase's todo list

On musl libc, ISO-2022-JP encoder is too eager to switch back to
1 byte encoding, musl's iconv always switch back after every combining
character. Comparing glibc and musl's output for this command
$ sed q t/t3900/ISO-2022-JP.txt| iconv -f ISO-2022-JP -t utf-8 |
	iconv -f utf-8 -t ISO-2022-JP | xxd

glibc:
00000000: 1b24 4224 4f24 6c24 5224 5b24 551b 2842  .$B$O$l$R$[$U.(B
00000010: 0a                                       .

musl:
00000000: 1b24 4224 4f1b 2842 1b24 4224 6c1b 2842  .$B$O.(B.$B$l.(B
00000010: 1b24 4224 521b 2842 1b24 4224 5b1b 2842  .$B$R.(B.$B$[.(B
00000020: 1b24 4224 551b 2842 0a                   .$B$U.(B.

Although musl iconv's output isn't optimal, it's still correct.

From commit 7d509878b8, ("pretty.c: format string with truncate respects
logOutputEncoding", 2014-05-21), we're encoding the message to utf-8
first, then format it and convert the message to the actual output
encoding on git commit --squash.

Thus, t3900::test_commit_autosquash_flags is failing on musl libc.

Reencode to utf-8 before arranging rebase's todo list.

By doing this, we also remove a breakage noticed by a test added in the
previous commit.

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>