diff3: Add support for -m

diff3 in -m mode generates a complete file with changes bracketed with
conflict markers. This adds support for diff3 to generate version
control style three way merge output.

The output format was inferred from looking at the gnu diff3 output on a
selection of test files as a specification of what diff3 -m should
output is not available. It is likely there are cases where the -m
output differs from other tools and I am happy to update diff3 to
address these.

Discussed with: pstef, kevans
Sponsored by: Klara, Inc.