phabricator: add node and p1 to hg:meta property
The "hg:meta" property is for extra metadata to reconstruct the patch.
Previously it does not have node or parent information since I think by
reading the patch again, the commit message will be mangled (like, added the
"Differential Revision" line) and we cannot preserve the commit hash.
However, the "parent" information could be useful. It could be helpful to
locate the "base revision" so in case of a conflict applying the patch
directly, we might be able to use 3-way merge to resolve it correctly.
Note: "local:commits" is an existing "property" used by Phabricator that has
the node and parent information. However, it lacks of timezone information
and requires "author" and "authorEmail" to be separated. So we are using a
different "property" - "hg:meta" to be distinguished from "local:commits".