The Bitbucket post-commit support was failing to generate a suitable

diff for display. When fetching a commit, the base_commit_id field was

being set to the ID of the commit, rather than the parent commit. This

led to fetching the patched version of the files for the diff, rather

than the source files.
The Commit.base_commit_id field is deprecated and redundant these days.

We instead want to use Commit.parent. This change updates everything to

use that, and passes it through as the base commit ID for file

fetches.
Deprecation docs have also been updated accordingly.

Unit tests pass.
Manually tested post-commit on Bitbucket and GitLab (the two that both

support post-commit and use base_commit_id.