R1:6bcfc212ebd4

Allow diff change detection to complete for Mercurial changes which remove a binary file

Summary:
See PHI1044. When you push a commit, we try to detect if the diff is the same as the last diff in Differential. This is a soft/heuristic check and only used to provide a hint in email ("CHANGED SINCE LAST DIFF").

For some changes, we can end up on this pathway with a binary changeset for a deleted file in the commit, and try to fetch the file data. This will fail since the file has been deleted. I'm not…