tempfile: remove deactivated list entries

Once a "struct tempfile" is added to the global cleanup
list, it is never removed. This means that its storage must
remain valid for the lifetime of the program. For single-use
tempfiles and locks, this isn't a big deal: we just declare
the struct static. But for library code which may take
multiple simultaneous locks (like the ref code), they're
forced to allocate a struct on the heap and leak it.

This is mostly OK in practice. The size of the leak is
bounded by the number of refs, and most programs exit after
operating on a fixed number of refs (and allocate
simultaneous memory proportional to the number of ref
updates in the first place). But:

  1. It isn't hard to imagine a real leak: a program which
     runs for a long time taking a series of ref update
     instructions and fulfilling them one by one. I don't
     think we have such a program now, but it's certainly
     plausible.

  2. The leaked entries appear as false positives to
     tools like valgrind.

Let's relax this rule by keeping only "active" tempfiles on
the list. We can do this easily by moving the list-add
operation from prepare_tempfile_object to activate_tempfile,
and adding a deletion in deactivate_tempfile.

Existing callers do not need to be updated immediately.
They'll continue to leak any tempfile objects they may have
allocated, but that's no different than the status quo. We
can clean them up individually.

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

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