The current version of the commit creation and amend function are unsafe
to use when passing the update_ref parameter, as they do not check that
the reference at the moment of update points to what the user expects.

Make sure that we're moving history forward when we ask the library to
update the reference for us by checking that the first parent of the new
commit is the current value of the reference. We also make sure that the
ref we're updating hasn't moved between the read and the write.

Similarly, when amending a commit, make sure that the current tip of the
branch is the commit we're amending.

This finds a short id string that will unambiguously select the
given object, starting with the core.abbrev length (usually 7)
and growing until it is no longer ambiguous.

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.

Some tests of paths that can't actually be written to disk

Leftover OID -> ID changes

A lot of the tests were checking for overflow, which we don't have
anymore, so we can remove them.

This was not converted when we converted the rest, so do it now.