Fix metadata rendering for files moves, etc.

Summary:
Some changeset metadata was not being correctly passed between the top-level
parser and the subparser, so it would be lost or incorrect when rendering
headers like "This file was moved from x to y." or rendering certain content
shields, like "the contents of this file were not modified".

Test Plan:
Created a new diff with a file move in it, rendered it, saw "This file was moved
from README to READYOU" correctly.

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, grglr, aran
CC: aran, epriestley
Differential Revision: 321