difftool: remove an unnecessary call to strbuf_release()
The `buf` strbuf is reused again later in the same function, so there
is no benefit to calling strbuf_release(). The subsequent usage is
already using strbuf_reset() to reset the buffer, so releasing it
early is only going to lead to a wasteful reallocation.
Remove the early call to strbuf_release(). The same strbuf is already
cleaned up in the "finish:" section so nothing is leaked, either.
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>