templater: consistently join() string-like object per character (BC)
The old behavior was copied from join() of a lazy generator string, which
was unified to the behavior of join() of a byte string by the previous patch.
This patch fixes the mappable type to do the same.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}