sha256 base32 value of a git checkout

[Pierre-Henry F.]

Hello,

I'm trying my hand on defining a package.
I have a source defined as in:

(origin

  (method git-fetch)

  (uri "some/git/dir")

  (commit "acommit")

  (file-name "a_filename")

  (sha256 (base32 "XXX")))

when origin resolves to an archive, guix download $url gives the value of "XXX"
but when using git-fetch... how to get the value of "XXX" ?

Thanks,
phf