git-log produces no output

git-log produces no output

When $PAGER is set to 'less -i', we used to fail because we
assumed the $PAGER is a command and simply exec'ed it.

Try exec first, and then run it through shell if it fails.  This
allows even funkier PAGERs like these ;-):

	PAGER='sed -e "s/^/`date`: /" | more'
	PAGER='contrib/colordiff.perl | less -RS'

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>