The usefulness of these helpers came up for me while debugging
some of the iterator changes that I was making, so since they
have also been requested (albeit indirectly) I thought I'd include
them.

The basic structure of each function is courtesy of arrbee.

This adds an API to amend an existing commit, basically a shorthand
for creating a new commit filling in missing parameters from the
values of an existing commit.  As part of this, I also added a new
"sys" API to create a commit using a callback to get the parents.
This allowed me to rewrite all the other commit creation APIs so
that temporary allocations are no longer needed.

We don't actually pass the old value yet.

Add a parameter to the backend to allow checking for the old symbolic
target.

Allow updating references if the old value matches the given one.