Q:

Is it possible to `git add` a file currently protected by `assume-unchanged`?

I have certain files that I rarely want new versions committed on but change often due to IDE settings.
git update-index --assume-unchanged meta.xml

Is it possible to forcibly git add meta.xml without having to apply no-assume-unchanged first?

A:

It doesn't seem possible: both git update-index --assmue-unchanged and git update-index --skip-worktree would make a git add impossible.
That leaves you with the definition of an alias, which would:
git update-index --no-assume-unchanged
git add -- theFile
git update-index --assume-unchanged

Something along the lines of:
addf = "!f() { git update-index --no-assume-unchanged -- $1; git add -- $1; git update-index --assume-unchanged -- $1}; f

