git config --local receive.denyCurrentBranch updateInstead
https://github.com/git/git/blob/v2.3.0/Documentation/config.txt#L2155
Use that on the server repository, and it also updates the working tree if no untracked overwrite would happen.
It was added in Git 2.3 as mentioned by VonC in the comments.
I've compiled Git 2.3 and gave it a try. Sample usage:
git init server
cd server
touch a
git add .
git commit -m 0
git config --local receive.denyCurrentBranch updateInstead
cd ..
git clone server local
cd local
touch b
git add .
git commit -m 1
git push origin master:master
cd ../server
ls
Output:
a
b
Yay, b
got pushed!