templatefilters: add commonprefix
The commonprefix filter takes a list of files names like files() and
returns the longest directory name common to all elements.
Differential Revision: https://phab.mercurial-scm.org/D3439
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}