[PATCH] Finish making --emu23 equivalent to pure 2-way merge.

[PATCH] Finish making --emu23 equivalent to pure 2-way merge.

This adds #3ALT rule (and #2ALT rule for symmetry) to the
read-tree 3-way merge logic that collapses paths that are added
only in one branch and not in the other internally.

This makes --emu23 to succeed in the last remaining case where
the pure 2-way merge succeeded and earlier one failed.  Running
diff between t1001 and t1005 test scripts shows that the only
difference between the two is that --emu23 can leave the states
into separate stages so that the user can use usual 3-way merge
resolution techniques to carry forward the local changes when
pure 2-way merge would have refused to run.

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