Remove suggestion to undo a commit

When a problem is detected in post-commit we suggested the user to
either amend it or to undo it. However, undoing the commit is hardly
the best solution. So, I'm removing that suggestion here.
Fix CheckJira's commit-msg error message

Since in the commit-msg hook we still don't have a complete commit the
error messages shouldn't mention its id.
Fix config compatibilization

The _compatibilize_config function was inadvertently creating the
githooks.checkacls and githooks.checkjira sections.
Make githooks.plugin accept a list of plugins

The githooks.plugin configuration variable now accepts a
space-separated list of plugin names, which makes it a little easier
to specify lots of plugins.
v0.047

  [Changes]
  - Remove suggestion to undo a commit when a problem is detected in
    the post-commit hook. Now we only suggest to amend it.

  - Make githooks.plugin configuration variable accept a list of
    plugins names.

  [Fixes]
  - Fix handling of fully qualified module names for plugins.

  - Fix CheckJira's commit-msg error message which was incorrectly
    showing a commit SHA-1 as '0000000'.