[volt build] Show warning when HEAD and locked revision are different #134

If a user run git pull in $VOLTPATH/repos/<repos>,
volt build still installs old files when build.strategy = "copy" in $VOLTPATH/config.toml
because it extracts files from git objects of lock.json revision.

[WARN] {repos}: HEAD and lock.json revision are different
[WARN]   HEAD: {head revision}
[WARN]   locked revision: {lock.json revision}
[WARN]   Please run 'volt get {repos}' to update locked revision.

Even when build.strategy = "symlink", difference between HEAD and lock.json revision is undesirable situation.
Should show warnings both when build.strategy = "copy", and build.strategy = "symlink".