We truncate hunk-header line at 80 bytes, but that 80th byte
could be in the middle of a character, which is bad.  This uses
pick_one_utf8_char() function to make sure we do not cut a character
in the middle.

This assumes that the internal representation of the text is
UTF-8.  This needs to be extended in the future but the optimal
direction has not been decided yet.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>