t3900: demonstrate git-rebase problem with multi encoding

t3900: demonstrate git-rebase problem with multi encoding

We're using fixup!/squash! <subject> to mark if current commit will be
used to be fixed up or squashed to a previous commit.

However, if we're changing i18n.commitencoding after making the
original commit but before making the fixing up, we couldn't find the
original commit to do the fixup/squash.

Add a test to demonstrate that problem.

Helped-by: Jeff King <redacted>
Signed-off-by: Doan Tran Cong Danh <redacted>
Signed-off-by: Junio C Hamano <redacted>