Compilation GIT_BUF_INIT error ?

[zimoun]

Dear,

I am running, more or less updated, Debian with libgit2-dev (and dependencies) installed. Then, R --vanilla is launched, and install.packages("git2r2") using some CRAN mirror fails with this message:

git2r_branch.c:407:19: error: ‘GIT_BUF_INIT’ undeclared (first use in this function)
     git_buf buf = GIT_BUF_INIT;

I do not see what I am missing and what is going wrong ?

(to be honest, I am only interesting by the devtools package, and since git2r is one dependency, I am blocked)

Thank you in advance for any help or advice.