Don't overwrite modifications to ModificationTimestampField

Review Request #9816 — Created March 21, 2018 and submitted — Latest diff uploaded

Information

Djblets
release-1.0.x
a0aee27...

Reviewers

The ModificationTimestampField now tracks manual modifications to the
value so that if code manually sets a timestamp it will not be
overwritten during model save. This allows models to manage timestamps
manually in some cases without having to resort to mucking about with
save_base, etc.

Ran unit tests.