Retain author information with --goal=rebase

Keeps GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE.
Merge branch 'knirch/preserve-author-info'

When simplifying an incremental merge into a "rebase", preserve the
author info from the original commits (but update the committer info
to the person running "git-imerge simplify").

I've taken some liberties when merging this commit, integrating it
into the commit_tree() command that I recently added for backwards
compatibility reasons, and renaming the function get_commit_info() to
get_author_info().

Conflicts:
	git-imerge