submodule--helper: use strbuf_release() to free strbufs

submodule--helper: use strbuf_release() to free strbufs

The prepare_to_clone_next_submodule() function has a few local-variable
strbufs. We use strbuf_reset() throughout the function to reuse the
buffers over and over. But at the end of the function we also use
strbuf_reset() as they go out of scope, which means we end up leaking
their heap buffers. This should be strbuf_release() instead.

These were introduced by 48308681b0 (git submodule update: have a
dedicated helper for cloning, 2016-02-29), but it doesn't seem to have
the same mistake elsewhere. Likewise, I looked for other instances of
the pattern in the submodule--helper file but couldn't find any.

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