Retain author information with --goal=rebase

Keeps GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE.
Complete web links.

(If a web log is called a "blog", is a web link called a "blink"?)
Fix Python 2.6.x compatibility.

The subprocess module in Python 2.6 has a CalledProcessError class,
but its constructor doesn't take a third (output) argument.  So don't
only pass the constructor two arguments.
Extract a method commit_tree().

Its implementation needs to be changed, so centralized how it is called.
Pipe commit message into "git commit-tree" rather than using "-m" option.

The "-m" option was added to Git recently, so use the old-fashioned
method to be portable to older Git versions.
Merge branch 'commit-tree-no-m'

Do not use "git commit-tree -m", because the "-m" option has only
recently been added to Git.
Fix call sequence for old "git commit-tree".

Apparently old "git commit-tree" requires "TREE" *before* "-p PARENT".
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