bookmarks: write bookmarks file deterministically
Bookmarks are internally stored as an unsorted dictionary. Let's at
least write out the bookmarks file in a deterministic order so it is
easier to test, diff, etc.
Differential Revision: https://phab.mercurial-scm.org/D2469
#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