commit-tree: encourage UTF-8 commit messages.

commit-tree: encourage UTF-8 commit messages.

Introduce is_utf() to check if a text looks like it is encoded
in UTF-8, utf8_width() to count display width, and implements
print_wrapped_text() using them.

git-commit-tree warns if the commit message does not minimally
conform to the UTF-8 encoding when i18n.commitencoding is either
unset, or set to "utf-8".

Signed-off-by: Junio C Hamano <junkio@cox.net>