apply: move lockfile into `apply_state`

apply: move lockfile into `apply_state`

We have two users of `struct apply_state` and the related functionality
in apply.c. Each user sets up its `apply_state` by handing over a
pointer to its static `lock_file`. (Before 076aa2cbd (tempfile:
auto-allocate tempfiles on heap, 2017-09-05), we could never free
lockfiles, so making them static was a reasonable approach.)

Other than that, they never directly access their `lock_file`s, which
are instead handled by the functionality in apply.c.

To make life easier for the caller and to make it less tempting for a
future caller to mess with the lock, make apply.c fully responsible for
setting up the `lock_file`. As mentioned above, it is now safe to free a
`lock_file`, so we can make the `struct apply_state` contain an actual
`struct lock_file` instead of a pointer to one.

The user in builtin/apply.c is rather simple. For builtin/am.c, we might
worry that the lock state is actually meant to be inherited across
calls. But the lock is only taken as `apply_all_patches()` executes, and
code inspection shows that it will always be released.

Alternatively, we can observe that the lock itself is never queried
directly. When we decide whether we should lock, we check a related
variable `newfd`. That variable is not inherited, so from the point of
view of apply.c, the state machine really is reset with each call to
`init_apply_state()`. (It would be a bug if `newfd` and the lock status
were not in sync. The duplication of information in `newfd` and the lock
will be addressed in the next patch.)

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