relnotes: copy "next" to "5.2" and clear "next"
This is the same thing as we did for 5.1 in
cba59b338976 (relnotes:
copy "next" to "5.1" and clear "next", 2019-08-01).
Differential Revision: https://phab.mercurial-scm.org/D7231
#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