Merge branch 'sy/git-gui-i18n-ja' into maint

Update Japanese translation for "git-gui".

* sy/git-gui-i18n-ja:
  git-gui: update Japanese information
  git-gui: update Japanese translation
  git-gui: add Japanese language code
  git-gui: apply po template to Japanese translation
  git-gui: consistently use the same word for "blame" in Japanese
  git-gui: consistently use the same word for "remote" in Japanese