Fix 'gone' status when pulling an empty repo from the remote

[strboul]

Actual behavior

Pulling an empty repository from upstream creates a status "gone".

To reproduce:

mkdir /tmp/gs-test
cd /tmp/gs-test
mkdir bar-upstream && cd bar-upstream
git init --bare # remote must be 'bare'
cd ..
git clone bar-upstream bar
mkdir foo && cd foo
git init
cd ..
git-substatus
#  directory: </tmp/gs-test>
# • bar  master  gone
# • foo  master  <sync>
cd bar
git status -sb
## No commits yet on master...origin/master [gone]

Expected behavior

The status should instead be called <sync> because there are no changes.