Use git-var(1) to fetch GIT_EDITOR
Use git-var(1) to fetch GIT_EDITOR

Invoke "git var GIT_EDITOR" instead of duplicating its logic.

Signed-off-by: Stefan Hajnoczi <484520986196e4a799496429ce8a499eb6488adb@gmail.com>

stefanha/git-publish,stefanha/git-publish