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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif