tmp-objdir: introduce API for temporary object directories

tmp-objdir: introduce API for temporary object directories

Once objects are added to the object database by a process,
they cannot easily be deleted, as we don't know what other
processes may have started referencing them. We have to
clean them up with git-gc, which will apply the usual
reachability and grace-period checks.

This patch provides an alternative: it helps callers create
a temporary directory inside the object directory, and a
temporary environment which can be passed to sub-programs to
ask them to write there (the original object directory
remains accessible as an alternate of the temporary one).

See tmp-objdir.h for details on the API.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>