D4602: phabricator: move extension from contrib to hgext

[Yuya Nishihara]

Added workaround for test-check-help. as follows:

```
diff --git a/tests/test-check-help.t b/tests/test-check-help.t
--- a/tests/test-check-help.t
+++ b/tests/test-check-help.t
@@ -26,4 +26,4 @@ Check if ":hg:`help TOPIC`" is valid:
   $ testrepohg files 'glob:{hgdemandimport,hgext,mercurial}/**/*.py' \
   > | sed 's|\\|/|g' \
   > | xargs $PYTHON "$TESTTMP/scanhelptopics.py" \
-  > | xargs -n1 hg help > /dev/null
+  > | xargs -n1 hg help --config extensions.phabricator= > /dev/null
```

This doesn't look nice, but should be okay for now.