In commit b972f20c55, the diff parser was refactored to remove

DiffParser.get_orig_commit_id(). As aprt of that, a regression was

introduced into create_filediff() that meant whenever the parent revision

was an instance of Revision (rather than a str or bytes), we had a crash

in convert_to_unicode.
The parent revision can be a Revision object specifically whan it is

PRE_CREATION, such as when adding a new file to a Perforce repository.
This change re-introduces the coersion to string of the

parent_source_revision which was lost in the refactoring.
Fixes https://hellosplat.com/s/beanbag/tickets/4988/.

NOTE: A slightly different patch is required for RB5.
See also https://reviews.reviewboard.org/r/12799/

Re-do the steps that trigger the issue in https://hellosplat.com/s/beanbag/tickets/4988/. PASS
I tried to run the nose tests, and mostly they pass, but then the Perforce tests just hang. There seems to be some local setup required, but I couldn't find any info on setting up the environemnt that the tests expect.