[R-sig-Debian] c2d4u: missing dependency for r-cran-gert and r-cran-v8 on bionic

[Jeroen Ooms]

A user has reported, and I have confirmed, that r-cran-gert from c2d4u
is uninstallable on bionic because it is compiled against a version of
libgit2 that is not included with c2d4u:

 # apt-get install r-cran-gert
 The following packages have unmet dependencies:
   r-cran-gert : Depends: libgit2-27 (>= 0.26.0) but it is not installable
  E: Unable to correct problems, you have held broken packages.

>From the c2d4u buildlog I can see that it is compiling r-cran-gert
against the libgit2 backports from ppa:cran/libgit2. So I think the
easiest solution is to put a copy of that libgit2 into c2d4u.

The same seems to hold for r-cran-v8.

The problem does not exist for ubuntu:focal because here the stock
versions of libgit2 and libv8 are sufficient and we do not provide ppa
backports.