tools/node-modules: add a 'verify' command
tools/node-modules: add a 'verify' command

This new subcommand takes a reference (in the current repository) and
extracts the `package.json` and `node_modules/` from that commit.  It
then repeats the build of `node_modules/` and verifies that it reaches
the same result.

In the event that the result is different, `diff` output is produced to
show exactly what didn't match — we show a diffstat, and the diff of the
`package-lock.json` file: those should adequately describe just about
any change that may occur in "legitimate" circumstances.

martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit