Add git_commit_amend API

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.
Merge pull request #2056 from libgit2/rb/commit-amend

Add git_commit_amend API