Back in #2264, we made it possible to change the branch wikis use from
the hardcoded "master" branch to `[repository].DEFAULT_BRANCH`. However,
the API endpoints were not updated, and the "master" branch remained
hardcoded there.
This change fixes that, the API endpoints will now respect the
repository's `WikiBranch`.
Fixes#3391.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>