Added annex.freezecontent-command and annex.thawcontent-command configs

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

Test.hs

@@ -142,7 +142,9 @@ runner opts
 		exitWith exitcode
 	runsubprocesstests (Just _) = isolateGitConfig $ do
 		ensuretmpdir
-		crippledfilesystem <- fst <$> Annex.Init.probeCrippledFileSystem' (toRawFilePath tmpdir)
+		crippledfilesystem <- fst <$> Annex.Init.probeCrippledFileSystem'
+			(toRawFilePath tmpdir)
+			Nothing Nothing
 		adjustedbranchok <- Annex.AdjustedBranch.isGitVersionSupported
 		case tryIngredients ingredients (tastyOptionSet opts) (tests crippledfilesystem adjustedbranchok opts) of
 			Nothing -> error "No tests found!?"