cache-tree: share code between functions writing an index as a tree

write_tree_from_memory() appeared to be a merge-recursive special that
basically duplicated write_index_as_tree().  The two have a different
signature, but the bigger difference was just that write_index_as_tree()
would always unconditionally read the index off of disk instead of
working on the current in-memory index.  So:

  * split out common code into write_index_as_tree_internal()

  * rename write_tree_from_memory() to write_inmemory_index_as_tree(),
    make it call write_index_as_tree_internal(), and move it to
    cache-tree.c

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -760,7 +760,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			 */
 			init_merge_options(&o, the_repository);
 			o.verbosity = 0;
-			work = write_tree_from_memory(&o);
+			work = write_in_core_index_as_tree(the_repository);
 
 			ret = reset_tree(new_tree,
 					 opts, 1,