Deduplicate code by moving tree_desc initialization into a helper
function, fill_tree_desc_strict. It is like fill_tree_descriptor,
except that it only accepts tree hashes and no tree references (tags,
commits). No functional change.
Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>