pretty: remove reencode_commit_message()

pretty: remove reencode_commit_message()

This function has only two callsites, and is a thin wrapper whose
usefulness is dubious.  When the caller needs to learn the log
output encoding, it should be able to do so by directly calling
get_log_output_encoding() and calling the underlying
logmsg_reencode() with it.

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