tree: simplify parse_tree_indirect()

tree: simplify parse_tree_indirect()

Reduce code duplication by turning parse_tree_indirect() into a wrapper
of repo_peel_to_type().  This avoids a segfault when handling a broken
tag where ->tagged is NULL.  The new version also checks the return
value of parse_object() that was ignored by the old one.

Initial-patch-by: Stefan Sperling <stsp@stsp.name>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>