Arcyd: report on diff reduction attempts

Improve the user experience when working with very large diffs, add
detail on measures taken to reduce the size of diffs.

Bubble 'reduction techniques' used to reduce diff sizes up to the repo
processor and supply to abdcmnt_commenter as warnings for the user.

Test Plan:
updated abdt_differ__t to check for reductions
$ ./precommit.sh

inspect comments left on big added file
$ ./testbed/arcyd/acceptance_tests.sh
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