sparse-checkout: use fdopen_lock_file() instead of xfdopen()

sparse-checkout: use fdopen_lock_file() instead of xfdopen()

When updating sparse patterns, we open a lock_file to write out the new
data. The lock_file struct holds the file descriptor, but we call
fdopen() to get a stdio handle to do the actual write.

After we finish writing, we fflush() so that all of the data is on disk,
and then call commit_lock_file() which closes the descriptor. But we
never fclose() the stdio handle, leaking it.

The obvious solution seems like it would be to just call fclose(). But
when? If we do it before commit_lock_file(), then the lock_file code is
left thinking it owns the now-closed file descriptor, and will do an
extra close() on the descriptor. But if we do it before, we have the
opposite problem: the lock_file code will close the descriptor, and
fclose() will do the extra close().

We can handle this correctly by using fdopen_lock_file(). That leaves
ownership of the stdio handle with the lock_file, which knows not to
double-close it.

We do have to adjust the code a bit:

  - we have to handle errors ourselves; we can just die(), since that's
    what xfdopen() would have done (and we can even provide a more
    specific error message).

  - we no longer need to call fflush(); committing the lock-file
    auto-closes it, which will now do the flush for us. As a bonus, this
    will actually check that the flush was successful before renaming
    the file into place.

  - we can get rid of the local "fd" variable, since we never look at it
    ourselves now

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