UTF-8: introduce i18n.logoutputencoding.

UTF-8: introduce i18n.logoutputencoding.

It is plausible for somebody to want to view the commit log in a
different encoding from i18n.commitencoding -- the project's
policy may be UTF-8 and the user may be using a commit message
hook to run iconv to conform to that policy (and either not have
i18n.commitencoding to default to UTF-8 or have it explicitly
set to UTF-8).  Even then, Latin-1 may be more convenient for
the usual pager and the terminal the user uses.

The new variable i18n.logoutputencoding is used in preference to
i18n.commitencoding to decide what encoding to recode the log
output in when git-log and friends formats the commit log message.

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