Problem creating commits/trees with commit-tree/mktree

Hello,

I want to write commits to a branch without touching the index or
having a checkout (for a git subcommand I'm writing).

I can create new blobs and trees but can't figure out how to commit a
new tree/blob _with_ the old tree.

Currently, I do something a lot like:

objsha=$(echo 'contents' | git hash-object -w --stdin)
objtreesha=$(printf "100644 blob $objsha\tfile.txt\000" | git mktree -z)
newtreesha=$(printf "040000 tree $objtreesha\ttreefileisin\000" | git mktree -z)
echo 'commit msg' | git commit-tree $newtreesha -p $(git rev-parse
refs/heads/new)

I get a commit with treefileisin/file.txt. I haven't included the
other trees/files so they are gone in this commit. How do I include
them? Is commit-tree the wrong tool?

Is there some way to use git ls-tree that I don't know about?

Gavin

-- 
Gavin Beatty

SEMPER UBI SUB UBI