lib/repo: Auto-recreate repo/tmp if it's deleted

We can accumulate a lot of space there; let's be nice to people who delete the
whole directory.

Closes: https://github.com/ostreedev/ostree/issues/1018

Closes: #1020
Approved by: jlebon

tests/basic-test.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..$((69 + ${extra_basic_tests:-0}))"
+echo "1..$((70 + ${extra_basic_tests:-0}))"
 
 $CMD_PREFIX ostree --version > version.yaml
 python -c 'import yaml; yaml.safe_load(open("version.yaml"))'
@@ -112,6 +112,14 @@ ostree_repo_init test-repo --mode=bare-user
 rm test-repo -rf
 echo "ok repo-init on existing repo"
 
+rm test-repo -rf
+ostree_repo_init test-repo --mode=bare-user
+${CMD_PREFIX} ostree --repo=test-repo refs
+rm -rf test-repo/tmp
+${CMD_PREFIX} ostree --repo=test-repo refs
+assert_has_dir test-repo/tmp
+echo "ok autocreate tmp"
+
 rm checkout-test2 -rf
 $OSTREE checkout test2 checkout-test2
 cd checkout-test2