templater: add shorthand for building a dict like {"key": key}
Like field init shorthand of Rust. This is convenient for building a JSON
object from selected keywords.
This means dict() won't support Python-like dict(iterable) syntax because
it's ambiguous. Perhaps it could be implemented as 'mapdict(xs % (k, v))'.
#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
foobar
#endif