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 is failing on Linux with musl libc.

This problem wasn't specific to musl libc. On Linux with glibc, this
problem can be observed by:

for encoding in utf-8 iso-8859-1; do
	# commit using the encoding
	echo $encoding >file && git add file
	echo "éñcödèd with $encoding" | iconv -f utf-8 -t $encoding |
	  git -c i18n.commitEncoding=$encoding commit -F -
	# and then fixup without it
	echo "$encoding fixed" >file && git add file
	git commit --fixup HEAD
done
git rebase -i --autosquash --root

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

Signed-off-by: Doan Tran Cong Danh <redacted>
---
The code that demonstrate the problem on Linux with glibc is written by Jeff.
But I don't know how to attribute him properly.

 sequencer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sequencer.c b/sequencer.c
index 9d5964fd81..69430fe23f 100644
--- a/sequencer.c
+++ b/sequencer.c

@@ -5169,7 +5169,7 @@ int todo_list_rearrange_squash(struct todo_list *todo_list)
 		*commit_todo_item_at(&commit_todo, item->commit) = item;
 
 		parse_commit(item->commit);
-		commit_buffer = get_commit_buffer(item->commit, NULL);
+		commit_buffer = logmsg_reencode(item->commit, NULL, "UTF-8");
 		find_commit_subject(commit_buffer, &subject);
 		format_subject(&buf, subject, " ");
 		subject = subjects[i] = strbuf_detach(&buf, &subject_len);

-- 
2.24.0.rc2.296.geaf699fcc3