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))'.
#require test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
New errors are not allowed. Warnings are strongly discouraged.
$ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
> python contrib/check-config.py