git-rev-list: make --dense the default (and introduce "--sparse")

git-rev-list: make --dense the default (and introduce "--sparse")

This actually does three things:

 - make "--dense" the default for git-rev-list. Since dense is a no-op if
   no filenames are given, this doesn't actually change any historical
   behaviour, but it's logically the right default (if we want to prune on
   filenames, do it fully. The sparse "merge-only" thing may be useful,
   but it's not what you'd normally expect)

 - make "git-rev-parse" show the default revision control before it shows
   any pathnames.

   This was a real bug, but nobody would ever have noticed, because
   the default thing tends to only make sense for git-rev-list, and
   git-rev-list didn't use to take pathnames.

 - it changes "git-rev-list" to match the other commands that take a mix
   of revisions and filenames - it no longer requires the "--" before
   filenames (although you still need to do it if a filename could be
   confused with a revision name, eg "gitk" in the git archive)

This all just makes for much more pleasant and obvous usage. Just doing a

	gitk t/

does the obvious thing: it will show the history as it concerns the "t/"
subdirectory.

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