purge: prevent a silly crash with --confirm --files
if --files is passed, there was no directory to checks and `msg` was undefined.
This is now fixed and tested.
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint