cache-tree: do not lazy-fetch tentative tree

The cache-tree datastructure is used to speed up the comparison between the HEAD and the index, and when the index is updated by a cherry-pick (for example), a tree object that would represent the paths in the index in a directory is constructed in-core, to see if such a tree object exists already in the object store. When the lazy-fetch mechanism was introduced, we converted this "does the tree exist?" check into an "if it does not, and if we lazily cloned, see if the remote has it" call by mistake. Since the whole point of this check is to repair the cache-tree by recording an already existing tree object opportunistically, we shouldn't even try to fetch one from the remote. Pass the OBJECT_INFO_SKIP_FETCH_OBJECT flag to make sure we only check for existence in the local object store without triggering the lazy fetch mechanism. Signed-off-by: Jonathan Tan <jonathantanmy@google.com> [jc: rewritten the proposed log message] Signed-off-by: Junio C Hamano <gitster@pobox.com>