I thought this would be a relatively simple question since in upstream Git it's just git hash-object -w -t blob --stdin, but I can't seem to find the proper documentation to do this.

Given an arbitrary blob expressed as an io.Reader or a []byte, how can I store it into a repo, returning its hash?

The Storer interface does not seem to provide for this.

X-Y problem background: I'm writing a Git-based forge. I need a function that accepts an email patch and applies it on a newly created branch based on the HEAD of an on-disk bare repository. I already have functions that can parse the email patch and apply each file to a given source file. My current thought is to make a copy of the tree object of HEAD, iterate through the patch, taking source files from the tree as necessary then patching it then hash-object'ing the resulting blob into the repo, then editing the tree object to point towards these new blobs and ultimately committing the modified tree object on a new branch.