tempfile: release deactivated strbufs instead of resetting

tempfile: release deactivated strbufs instead of resetting

When a tempfile is deactivated, we reset its strbuf to the
empty string, which means we hold onto the memory for later
reuse.

Since we'd like to move to a system where tempfile structs
can actually be freed, deactivating one should drop all
resources it is currently using. And thus "release" rather
than "reset" is the appropriate function to call.

In theory the reset may have saved a malloc() when a
tempfile (or a lockfile) is reused multiple times. But in
practice this happened rarely. Most of our tempfiles are
single-use, since in cases where we might actually use many
(like ref locking) we xcalloc() a fresh one for each ref. In
fact, we leak those locks (to appease the rule that tempfile
storage can never be freed). Which means that using reset is
actively hurting us: instead of leaking just the tempfile
struct, we're leaking the extra heap chunk for the filename,
too.

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