We were checking for the existence of each file in a diff for existing

commits in a repository, which is unnecessary. We can trust that those

files exist, since if they didn't, the repository is corrupt and things

will fail anyway.
DiffSet.objects.create_from_data() now accepts a check_existence

parameter, which controls whether to perform file existence checks.

By default this is True, but it's set to False when updating a draft

from a commit.
This speeds things up considerably when posting existing commits for

review.

Tested posting commits for review. Saw that they were posted quickly,

compared to before.
Unit tests pass.