Be more careful about reference parsing

Be more careful about reference parsing

This does two things:

 - we don't allow "." and ".." as components of a refname. Thus get_sha1()
   will not accept "./refname" as being the same as "refname" any more.

 - git-rev-parse stops doing revision translation after seeing a pathname,
   to match the brhaviour of all the tools (once we see a pathname,
   everything else will also be parsed as a pathname).

Basically, if you did

	git log *

and "gitk" was somewhere in the "*", we don't want to replace the filename
"gitk" with the SHA1 of the branch with the same name.

Of course, if there is any change of ambiguity, you should always use "--"
to make it explicit what are filenames and what are revisions, but this
makes the normal cases sane. The refname rule also means that instead of
the "--", you can do the same thing we're used to doing with filenames
that start with a slash: use "./filename" instead, and now it's a
filename, not an option (and not a revision).

So "git log ./*.c" is now actually a perfectly valid thing to do, even if
the first C-file might have the same name as a branch.

Trivial test:

	git-rev-parse gitk ./gitk gitk

should output something like

	9843c3074dfbf57117565f6b7c93e3e6812857ee
	./gitk
	gitk

where the "./gitk" isn't seen as a revision, and the second "gitk" is a
filename simply because we've seen filenames already, and thus stopped
doing revision parsing.

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