Rename 'commit_parent' in 'commits' table to 'commit_previous_commit'.

Rename 'commit_parent' in 'commits' table to 'commit_previous_commit'.
https://bugs.webkit.org/show_bug.cgi?id=168816

Reviewed by Ryosuke Niwa.

Rename 'commit_parent' to avoid ambiguity in the coming feature.
For exisiting database, run
    "ALTER TABLE commits RENAME commit_parent TO commit_previous_commit;"
to update the database.

* init-database.sql:
* public/api/report-commits.php:
* public/include/commit-log-fetcher.php:
* server-tests/api-commits.js:
(then):
* server-tests/api-report-commits-tests.js:
(then):
* tools/sync-commits.py:
(main):
(Repository.fetch_commits_and_submit):
(GitRepository._revision_from_tokens):
* unit-tests/analysis-task-tests.js:
(sampleAnalysisTask):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@212947 268f45cc-cd09-0410-ab3c-d52691b4dbfc