R1:d016cac91513

Modularize parser for "Closes task X as Y"

Summary:
Ref T3886. Ref T3872. Ref T1812. We have several parsers which look for textual references to other objects, like:

Closes Tx.
Depends on Dy.
Reverts Dz.

Currently, these are pretty hard coded, don't get all the edge cases right, and don't generalize well. They're also implemented in the middle of Differential's field code. So I want to:

- Share more code so that, e.g., "Tx, Ty" always works (only some rules support it right now);
- fix bugs…