added hooks corresponding to annex.*-command

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