Refactor cache_tree_update idiom from commit

We'll need to safely create or update the cache-tree data of the_index
from other places.  While at it, give it an argument that lets us
silence the messages produced by unmerged entries (which prevent it
from working).

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 8f2bebe..4f782a3 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -862,10 +862,7 @@
 	 */
 	discard_cache();
 	read_cache_from(index_file);
-	if (!active_cache_tree)
-		active_cache_tree = cache_tree();
-	if (cache_tree_update(active_cache_tree,
-			      active_cache, active_nr, 0, 0) < 0) {
+	if (update_main_cache_tree(0)) {
 		error(_("Error building trees"));
 		return 0;
 	}