revlog: overwrite revlog config through copy of the config object
The new objects allow for this kind of blanket approach that make things
cleaner.
If we have more cases, it would probably deserve a context manager, but since we
only have two usage, I don't think it is worth it.
#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