builtin/gc: don't peek into `struct lock_file`

builtin/gc: don't peek into `struct lock_file`

A `struct lock_file` is pretty much just a wrapper around a tempfile.
But it's easy enough to avoid relying on this. Use the wrappers that the
lock file API provides rather than peeking at the temp file or even into
*its* internals.

Signed-off-by: Martin Ågren <redacted>
Reviewed-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>