read-cache: try not to peek into `struct {lock_,temp}file`

read-cache: try not to peek into `struct {lock_,temp}file`

Similar to the previous commits, try to avoid peeking into the `struct
lock_file`. We also have some `struct tempfile`s -- let's avoid looking
into those as well.

Note that `do_write_index()` takes a tempfile and that when we call it,
we either have a tempfile which we can easily hand down, or we have a
lock file, from which we need to somehow obtain the internal tempfile.
So we need to leave that one instance of peeking-into. Nevertheless,
this commit leaves us not relying on exactly how the path of the
tempfile / lock file is stored internally.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>