Support setting diff description via --comment

[Rob--W]

The diff description is the "Description" column of the diff history view in Phabricator. Currently, that field is always empty. In contrast, when a patch is uploaded via arc (Arcanist, the official tool from Phabricator), the value can be set (and defaults to "Revision updated.").

This PR adds the ability to set that comment in phlay with the --phlay flag.

See the commit message for more details.

I've verified this feature with the following scenarios:

• Update diff: https://phabricator.services.mozilla.com/D34277#1016468
• Update diff that was not owned by me: https://phabricator.services.mozilla.com/D31778#1016026
• Update diff and other metadata (summary, parent, BMO ID): https://phabricator.services.mozilla.com/D31496#1016747

This is just a small selection of the patches that I uploaded in the past two weeks, using this feature.

[mystor]

Thanks!