commit/commit-tree: correct latin1 to utf-8

commit/commit-tree: correct latin1 to utf-8

When a line in the message is not a valid utf-8, "git mailinfo"
attempts to convert it to utf-8 assuming the input is latin1 (and
punt if it does not convert cleanly).  Using the same heuristics in
"git commit" and "git commit-tree" lets the editor output be in
latin1 to make the overall system more consistent.

Signed-off-by: Junio C Hamano <gitster@pobox.com>