After thinking about it more, I realized that much of the change
I did on top of Linus' version does not make much sense.  This
commit reverts it so that it by default shows diffs with stage0
paths or stage2 paths with working tree; the unmerged stage to
use can be overridden with -1/-2/-3 option (-2 is the default so
essentially is a no-op).

When the index file is unmerged, we are by definition in the
middle of a conflicting merge, and we should show the diff with
stage 2 by default.  More importantly, paths without conflicts
are updated in the working tree and collapsed to stage0 in the
index, so showing diff with stage0 at the same time does not
hurt.  In normal cases, stage0 entries should be in sync with
the working tree files and does not clutter the output.  It even
helps the user to realize that the working tree has local
changes unrelated to the merge and remember to be careful not to
do a "git-commit -a" after resolving the conflicts.

When there is no unmerged entries, giving diff_unmerged_stage a
default value of 2 does not cause any harm, because it would not
be used anyway.  So in all, always showing diff between stage0
paths and unmerged entries from a stage (defaulting to 2) is the
right thing to do, as Linus originally did.

Signed-off-by: Junio C Hamano <junkio@cox.net>