[BUG] git --work-tree=... status

Hello,

I don't have time now to look into it, so for now as usual just a report. If nobody takes it, I will do it later.

	zeisberg@cassiopeia:~/gsrc/git$ git version
	git version 1.5.3.rc4.887.gd56871c

[That's Junio's next + my doc change "checkout-index doc: use --work-dir in the export example"]

	zeisberg@cassiopeia:~/gsrc/git$ mkdir export

	zeisberg@cassiopeia:~/gsrc/git$ git --work-tree=export checkout-index -a

	zeisberg@cassiopeia:~/gsrc/git$ git --work-tree=export status
	fatal: /home/zeisberg/usr/bin/git-status cannot be used without a working tree.

runstatus works as expected (at least after some thought):

	zeisberg@cassiopeia:~/gsrc/git$ git --work-tree=export runstatus | head
	# On branch next
	# Changed but not updated:
	#   (use "git add <file>..." to update what will be committed)
	#
	#       modified:   .gitignore
	#       modified:   .mailmap
	#       modified:   COPYING
	#       modified:   Documentation/.gitignore
	#       modified:   Documentation/Makefile
	#       modified:   Documentation/RelNotes-1.5.0.1.txt

The problem seems to be that require_work_tree demands more that I think it should. It calls `git rev-parse --is-inside-work-tree`.

Best regards Uwe