Raw commands to add object to existing tree

Both the community git manual: http://book.git-scm.com/7_raw_git.html

and Pro Git book: http://progit.org/book/ch9-2.html

walk you though low level details of creating objects and trees in git, and then committing these new trees.

However, they do not show you how to add and commit into existing trees.

Speaking in terms of files, I have a web application that needs to replace a file in a known directory in a known branch of a bare repository. I know the SHA1s of the directory tree and file. I want to commit directly into a bare repository to avoid having to manage working directories for different branches from the web application.

Thanks! David