phabricator: add tests of templatekeyword
Having tests is paying off: I found a bug and now it'll be easy to
fix!
Differential Revision: https://phab.mercurial-scm.org/D4603
phabricator: move extension from contrib to hgext
It's well-enough tested now and widely enough used I think we should
ship it.
Differential Revision: https://phab.mercurial-scm.org/D4602