Optimize the three-way merge of git-read-tree

As mentioned, the three-way case *should* be as trivial as the
following. It passes all the tests, and I verified that a conflicting
merge in the 100,000 file horror-case merged correctly (with the conflict
markers) in 0.687 seconds with this, so it works, but I'm lazy and
somebody else should double-check it [jc: followed all three-way merge
codepaths and verified it removes when it should].

Without this patch, the merge took 8.355 seconds, so this patch
really does make a huge difference for merge performance with lots and
lots of files, and we're not talking percentages, we're talking
orders-of-magnitude differences!

Now "unpack_trees()" is just fast enough that we don't need to avoid it
(although it's probably still a good idea to eventually convert it to use
the traverse_trees() infrastructure some day - just to avoid having
extraneous tree traversal functions).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -667,7 +667,6 @@ int threeway_merge(struct cache_entry **stages,
 	int no_anc_exists = 1;
 	int i;
 
-	remove_entry(remove);
 	for (i = 1; i < o->head_idx; i++) {
 		if (!stages[i] || stages[i] == o->df_conflict_entry)
 			any_anc_missing = 1;
@@ -730,8 +729,10 @@ int threeway_merge(struct cache_entry **stages,
 	}
 
 	/* #1 */
-	if (!head && !remote && any_anc_missing)
+	if (!head && !remote && any_anc_missing) {
+		remove_entry(remove);
 		return 0;
+	}
 
 	/* Under the new "aggressive" rule, we resolve mostly trivial
 	 * cases that we historically had git-merge-one-file resolve.
@@ -763,6 +764,7 @@ int threeway_merge(struct cache_entry **stages,
 		if ((head_deleted && remote_deleted) ||
 		    (head_deleted && remote && remote_match) ||
 		    (remote_deleted && head && head_match)) {
+			remove_entry(remove);
 			if (index)
 				return deleted_entry(index, index, o);
 			else if (ce && !head_deleted)
@@ -785,6 +787,7 @@ int threeway_merge(struct cache_entry **stages,
 		verify_uptodate(index, o);
 	}
 
+	remove_entry(remove);
 	o->nontrivial_merge = 1;
 
 	/* #2, #3, #4, #6, #7, #9, #10, #11. */