R1:3d3e63e4fc31

Fix an issue with empty 'Summary' fields in commit messages

Summary:
If the 'Summary' is not present and not inferred to be empty from a newline after the title with no explicit 'Summary' field, we'll copy all the field values from the revision (including NULL), not overwrite the 'Summary' value from the message (since it's not present) and then write the NULL back to the revision.

Instead, string cast the read from the Revision so we write back empty string in the not-provided case.

Test Plan: Ran "arc…