templater: abstract ifcontains() over wrapped types
This allows us to make .keytype private.
There's a minor BC that a hybrid dict/list of keytype=None now strictly
checks the type of the needle. For example, {ifcontains(rev, files)} no longer
matches a file named "1" at the rev=1. I made this change for consistency
with the get(dict, key) function. We can restore the old behavior by making
keytype=bytes the default if desired.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}