read-cache: fix leaking hashfile when writing index fails

read-cache: fix leaking hashfile when writing index fails

In `do_write_index()`, we use a `struct hashfile` to write the index
with a trailer hash. In case the write fails though, we never clean up
the allocated `hashfile` state and thus leak memory.

Refactor the code to have a common exit path where we can free this and
other allocated memory. While at it, refactor our use of `strbuf`s such
that we reuse the same buffer to avoid some unneeded allocations.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>