fix deadlock in restagePointerFiles

Fix a hang that occasionally occurred during commands such as move.
(A bug introduced in 10.20220927, in
commit 6a3bd283b8)

The restage.log was kept locked while running a complex index refresh
action. In an unusual situation, that action could need to write to the
restage log, which caused a deadlock.

The solution is a two-stage process. First the restage.log is moved to a
work file, which is done with the lock held. Then the content of the work
file is read and processed, which happens without the lock being held.
This is all done in a crash-safe manner.

Note that streamRestageLog may not be fully safe to run concurrently
with itself. That's ok, because restagePointerFiles uses it with the
index lock held, so only one can be run at a time.

streamRestageLog does delete the restage.old file at the end without
locking. If a calcRestageLog is run concurrently, it will either see the
file content before it was deleted, or will see it's missing. Either is
ok, because at most this will cause calcRestageLog to report more
work remains to be done than there is.

Sponsored-by: Dartmouth College's Datalad project

doc/bugs/git_annex_test_never_exits__63__/comment_9_230038504d75704baacd4d3ac750ee95._comment

@@ -0,0 +1,8 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 9"""
+ date="2022-12-08T18:07:47Z"
+ content="""
+I've developed a fix. Gonna re-run the reproducer long enough to be sure
+the bug is squashed..
+"""]]