Freeze first sets the file perms, and then runs
freezecontent-command. Thaw runs thawcontent-command before
restoring file permissions. This is in case the freeze command
prevents changing file perms, as eg setting a file immutable does.
Also, changing file perms tends to mess up previously set ACLs.
git-annex init's probe for crippled filesystem uses them, so if file perms
don't work, but freezecontent-command manages to prevent write to a file,
it won't treat the filesystem as crippled.
When the the filesystem has been probed as crippled, the hooks are not
used, because there seems to be no point then; git-annex won't be relying
on locking annex objects down. Also, this avoids them being run when the
file perms have not been changed, in case they somehow rely on
git-annex's setting of the file perms in order to work.
Sponsored-by: Dartmouth College's Datalad project
