No stop to edit for the new merge behavior of git

In Git 1.7.10, Merge will stop and wait for a merge commit log. This
backward-incompatible improvement that will break topgit. To fix it,
just export GIT_MERGE_AUTOEDIT=no.

See: http://git-blame.blogspot.jp/2012/02/anticipating-git-1710.html

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Robin Green <greenrd@greenrd.org>