merge script: tweak unmerged files message to match builtin

merge script: tweak unmerged files message to match builtin

Before:

	You are in the middle of a conflicted merge.

After:

	Merge is not possible because you have unmerged files.

I prefer the old message, but the new one is more consistent with
other commands and tests expect it.  In particular, without this
change the scripted merge does not pass t3030.

Based on v1.7.0-rc0~66^2 (Be more user-friendly when refusing to do
something because of conflict., 2010-01-12).

Cc: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>