Add helpers for syncing commit files with external testsuites

This provides "make sync-repos" to check out all locally available testsuites at
the commits indicated in their commit files, and "make sync-commits" to update
the commit files to the HEADs of the local testsuite repos.

Also adds the commit -> repo sync for the Makefile init target so initialization
always lands on the right version, and removes the corresponding explicit
checkout from the CI repo setup.

ci/init-external-repos.sh

@@ -13,7 +13,6 @@ set -e
 cd testing/external
 [[ ! -d zeek-testing ]] && make init
 cd zeek-testing
-git checkout -q $(cat ../commit-hash.zeek-testing)
 
 if [[ -n "${CIRRUS_CI}" ]]; then
     if [[ -d ../zeek-testing-traces ]]; then