grep: fix racy calls in grep_objects()

deref_tag() calls is_promisor_object() and parse_object(), both of which perform lazy initializations and other thread-unsafe operations. If it was only called by grep_objects() this wouldn't be a problem as the latter is only executed by the main thread. However, deref_tag() is also present in read_object_file()'s call stack. So calling deref_tag() in grep_objects() without acquiring the grep_read_mutex may incur in a race condition with object reading operations (such as the ones internally performed by fill_textconv(), called at fill_textconv_grep()). The same problem happens with the call to gitmodules_config_oid() which also has parse_object() in its call stack. Fix that protecting both calls with the said grep_read_mutex. Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>