We have a handful of methods on Repository that takes optional

arguments or options that control behavior, and these are allowed to be

passed as positional arguments. However, doing so makes some wrapping

functions a bit harder to deal with, as they may be dealing with subsets

of these arguments but need to take care to pass everything either as

positional or keyword arguments.
This has led to inconsistencies in unit tests as well, as we have many

spy functions that get parameter names wrong or make assumptions about

positional vs. keyword usage.
These methods have been updated to primarily take keyword-only

arguments. The two notable ones are get_file() and

get_file_exists(), which now require keyword arguments for everything.

As these methods have gotten more complex, and more combinations of

arguments are needed for file lookups (with deprecations planned for

base_commit_id), it's easiest just to ensure that these methods are

always called with explicit arguments going forward.
Positional arguments are still allowed until Review Board 9, but will

emit a deprecation warning.

All unit tests pass. No deprecation warnings were found.