store: refactor space delimited list to proper data structure
There is no good reason why are having a space delimited list and then using
`.split()` to get the actual list. Let's convert this into a proper collection.
Differential Revision: https://phab.mercurial-scm.org/D8908
#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