Use virtualenv for pre-commit script

.gitignore

@@ -1,3 +1,4 @@
+*.bak
 *.swp
 *~
 /venv/

pre-commit

@@ -8,6 +8,8 @@
 
 root="$(git rev-parse --show-toplevel 2>/dev/null)"
 venv="$root/venv"
+pip="$venv/bin/pip"
+python="$venv/bin/python"
 
 # Some sanity checking.
 hash python3 || exit 1
@@ -30,21 +32,20 @@ fi
 if [ ! -d "$venv" ]; then
     echo "Setting up python venv"
     python3 -m venv "$venv"
-    source "$venv/bin/activate"
-    pip3 install -r requirements.txt
-elif [ requirements.txt -nt "$venv" ]; then
-    source "$venv/bin/activate"
-    pip3 install --upgrade --upgrade-stategy eager -r requirements.txt
-else
-    source "$venv/bin/activate"
+fi
+
+source "$venv/bin/activate"
+
+if [ requirements.txt -nt "$venv" ]; then
+    "$pip" install --upgrade --upgrade-strategy eager -r requirements.txt
 fi
 
 exec 1>&2
 
 # Perform validation.
 trap 'git stash pop -q' EXIT
 git stash push -k -u -q -m "pre-commit stash from $(date '+%FT%T%z')"
-if ! python3 activities.py validate; then
+if ! "$python" activities.py validate; then
     echo "Validation failed; \`git commit --no-verify\` to suppress validation."
     exit 1
 fi