How do I do the reverse of gitk's "Write commit to file"?

No-one seems to have provided an actual answer to the question, so here's what I found from https://github.com/sinsunsan/archiref_wiki/wiki/Git-howto

patch -p1 < patch-file

This takes the patch file produced by 'Write commit to file', tells it to ignore the first level of path (-p1) i.e. a/, b/ in the file, and apply to the current files on disk. The result is then in your working tree, and can be committed as a new commit:

git commit -am'Commit message here please.'

I'm using Linux, but I think I see what you're talking about. In gitk there is a "Write commit to file" option when right-clicking a commit, which brings up a dialog that performs the command git diff-tree --stdin -p --pretty by default.

git apply is only for applying diffs, i.e. it won't create the commit object so that shouldn't be used. git am should be the correct tool for performing this operation, as it creates commit objects. However, it doesn't understand the format output by the above command, and creates the error you are seeing.

The easiest option is probably to create the patch using a format git am understands using git format-patch instead of git diff-tree. There may be a way to coerce git am into understanding the git diff-tree format, but I don't do patches much so am not aware of it offhand.