add temporary Breaks to force migration of libgit2

add temporary Breaks to force migration of libgit2

there's the following chain of migration-constraints:

libgit2 -> rust-libgit2-sys -> rust-git2 -> rust-cargo -> rust-debcargo
rust-debcargo --built-using--> rustc -> libgit2

that requires rustc and the libgit2 transition to migrate together.

but britney will attempt to trigger autopkgtests using rustc from
unstable (which in turn uses libgit2 1.8) for crates in testing. if
those crates use libgit2 themselves, then the pinning is incomplete and
an invalid combination is tested.

Signed-off-by: Fabian Grünbichler <debian@fabian.gruenbichler.email>