Find project root relative to the active buffer

Question

I can use the finddir function to find a parent folder that contains a .git folder (project root), like this:

let dir = finddir('.git/..', ';')

However, this only finds project roots that are parent folders of Vim's current working directory, which is usually the directory Vim was started in. I would like to find project roots that are parent folders of the location of the currently active buffer.

Edit: I tried

let dir = finddir('.git/..', expand('%').';')

But this always returns the current directory for some reason.

Answer 1

After following dedowsdi's instructions, it turns out you just need to do:

let dir = finddir('.git/..', expand('%:p:h').';')

expand(%) expands into the path of the current file (excluding the filename).