Improve git_changelog's handling of inconsistent commit orderings.

Improve git_changelog's handling of inconsistent commit orderings.

Use the CommitDate not the AuthorDate, as the former is representative of
the order in which things went into the main repository, and the latter
isn't very; we now have instances where the AuthorDate is as much as a
month before the patch really went in.  Also, get rid of the "commit order
inversions" heuristic, which turns out not to do anything very desirable.
Instead we just print commits in strict timestamp order, interpreting the
"timestamp" of a merged commit as its timestamp on the newest branch it
appears in.  This fixes some cases where very ancient commits were being
printed relatively early in the report.