subrepo: (mostly) use relative path in "skipping missing subrepository"
This is consistent with the other messages printed by these
functions. Note that addremove is a little different and prints
absolute (aka repo-relative) paths if no argument was given.
Differential Revision: https://phab.mercurial-scm.org/D5900
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}