tree.c: allow read_tree_recursive() to traverse gitlink entries

tree.c: allow read_tree_recursive() to traverse gitlink entries

When the callback function invoked from read_tree_recursive() returns
the value `READ_TREE_RECURSIVE` for a gitlink entry, the traversal will
now continue into the tree connected to the gitlinked commit. This
functionality can be used to allow inter-repository operations, but
since the current users of read_tree_recursive() does not yet support
such operations, they have been modified where necessary to make sure
that they never return READ_TREE_RECURSIVE for gitlink entries (hence
no change in behaviour should be introduces by this patch alone).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>