Add Japanese translation for GERRIT_CHANGE_COMMIT_MESSAGE help

Change-Id: I239d3d9b5b297141f406c9cc06adc6d6ac3e9d91
Minor updates in the Japanese help file

- Consistently capitalise "Gerrit"

- Add missing translation of `GERRIT_BRANCH` description.

- Add the "only if Gerrit version is 2.8 or higher" caveat on the
  description of `GERRIT_CHANGE_COMMIT_MESSAGE`.

Change-Id: Ie1e97cdb4cb04bc513e4ef9333a6e99c33036fec
Merge branch 'i' of gerrit-trigger-plugin