How to get Travis-CI build_number within after_script command

How can one get the build_number (and other build metadata) from within the after_script command in Travis-CI?

What have been tried already:

The documentation on build configuration says this, in the IRC notification section:

You also have the possibility to customize the message that will be sent to the channel(s) with a template:

notifications:
  irc:
    channels:
      - "irc.freenode.org#travis"
      - "irc.freenode.org#some-other-channel"
    template:
      - "%{repository} (%{commit}) : %{message} %{foo} "
      - "Build details: %{build_url}"

You can interpolate the following variables:

• repository: your GitHub repo URL.
• build_number: build number.
• branch: branch build name.
• commit: shorten commit SHA
• author: commit author name.
• message: travis message to the build.
• compare_url: commit change view URL.
• build_url: URL of the build detail.

Trying to get this to work within an after_script command as below, did not work at all:

language: java
after_script:
  - git commit -a -m "Committed by Travis-CI build number: %{build_number}"

It behaved as if .travis.yml file was absent/invalid (even though it did pass the Travis-CI YAML validation here).

It seems as though this should be doable, but could not find any sample that does this.

Could someone point me in the right direction?