These functions were originally conceived as wrapper
functions similar to xmkstemp(). They were later moved by 463db9b10 (wrapper: move odb_* to environment.c,
2010-11-06). The more appropriate place for a declaration is
in cache.h.
While we're at it, let's add some basic docstrings.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>