added hooks corresponding to annex.*-command

* Added freezecontent-annex and thawcontent-annex hooks that
  correspond to the git configs annex.freezecontent and
  annex.thawcontent.
* Added secure-erase-annex hook that corresponds to the git config
  annex.secure-erase-command.
* Added commitmessage-annex hook that corresponds to the git config
  annex.commitmessage-command.
* Added http-headers-annex hook that corresponds to the git config
  annex.http-headers-command.
  that correspond to the post-update-annex and pre-commit-annex hooks.

The use case for these is eg, setting up a git repository that is run in a
container, where the easiest way to provide a script is by putting it in
.git/hooks/, rather than copying it into the container in a way that puts
it in PATH.

This is all the ones that make sense to add for annex.*-config git configs.
annex.youtube-dl-command is not a hook, it's telling git-annex what command
to run. So is annex.shared-sop-command. So omitted those.

May later also want to add hooks corresponding to
`remote.<name>.annex-cost-command` etc.

Sponsored-by: the NIH-funded NICEMAN (ReproNim TR&D3) project

Annex/Content/LowLevel.hs

@@ -10,6 +10,7 @@
 module Annex.Content.LowLevel where
 
 import Annex.Common
+import Annex.Hook
 import Logs.Transfer
 import qualified Annex
 import Utility.DiskFree
@@ -25,11 +26,8 @@ import System.PosixCompat.Files (linkCount)
  - File may or may not be deleted at the end; caller is responsible for
  - making sure it's deleted. -}
 secureErase :: RawFilePath -> Annex ()
-secureErase file = maybe noop go =<< annexSecureEraseCommand <$> Annex.getGitConfig
-  where
-	go basecmd = void $ liftIO $
-		boolSystem "sh" [Param "-c", Param $ gencmd basecmd]
-	gencmd = massReplace [ ("%file", shellEscape (fromRawFilePath file)) ]
+secureErase = void . runAnnexPathHook "%file"
+	secureEraseAnnexHook annexSecureEraseCommand
 
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