tools: Don't lint in pre-commit hook if no files changed.

tools/pre-commit

@@ -9,11 +9,17 @@
 # contains your provisioned Zulip development environment, the linter
 # will automatically be run through `vagrant ssh`.
 
+changed_files=$(git diff --cached --name-only --diff-filter=ACM)
+if [ -z "$changed_files" ]; then
+    echo "No changed files to lint."
+    exit 0
+fi
+
 if [ -z "$VIRTUAL_ENV" ] && `which vagrant > /dev/null` && [ -e .vagrant ]; then
-    vcmd='/srv/zulip/tools/lint-all --pep8 --force $(cd /srv/zulip && git diff --cached --name-only --diff-filter=ACM) || true'
+    vcmd="/srv/zulip/tools/lint-all --pep8 --force $changed_files || true"
     echo "Running lint-all using vagrant..."
     vagrant ssh -c "$vcmd"
 else
-    ./tools/lint-all --pep8 --force $(git diff --cached --name-only --diff-filter=ACM) || true
+    ./tools/lint-all --pep8 --force $changed_files || true
 fi
 exit 0