Add GIT_MERGE_AUTOEDIT for git 1.8 support.

Summary:
From 'git help merge' (for git 1.8):

       --edit, --no-edit
           Invoke an editor before committing successful mechanical merge to
           further edit the auto-generated merge message, so that the user can
           explain and justify the merge. The --no-edit option can be used to
           accept the auto-generated message (this is generally discouraged).
           The --edit option is still useful if you are giving a draft message
           with the -m option from the command line and want to edit it in the
           editor.

           Older scripts may depend on the historical behaviour of not
           allowing the user to edit the merge log message. They will see an
           editor opened when they run git merge. To make it easier to adjust
           such scripts to the updated behaviour, the environment variable
           GIT_MERGE_AUTOEDIT can be set to no at the beginning of them.

We're an 'older scripts'!  We don't want to have to put in a
contentful merge message every time we merge from upstream
phabricator.

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2147