fix post-receive hook

tools/post-receive.example

@@ -12,7 +12,7 @@ while read old new refname; do
         (
             cd /home/codl/forget
             source venv/bin/activate
-            pip install -r requirements.txt
+            env -u GIT_DIR -u GIT_WORK_TREE pip install -r requirements.txt
             tools/write-version.sh
             FLASK_APP='forget.py' flask db upgrade
             if [[ $(git diff --name-only ${old}..${new} dodo.py) == 'dodo.py' ]]