abdt_differ: support huuge diffs by default

Increase the definition of
"largest amount of context conceivably needed".

One of the generated test data files is now > 10,000 lines, support
loading that into a review with full context.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
Arcyd: acceptance_tests - large file small change

Simulate making a small change to an enormous file to exercise the non
"very large change" path in the commenter.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
abdt_differ: support medium diff context amount

Introduce a third diff context setting so that it's not either
one extreme or the other.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh