ts-xen-build-prep: force use of git protocol v2

ts-xen-build-prep: force use of git protocol v2

Recent version of QEMU (to be 8.1) started to use meson subproject to
clone extra repo. With the example of the subproject "dtc", they do a
shallow clone with a sha1. Meson end up running:

    git fetch --depth 1 origin b6910bec11614980a21e46fbccc35934b671bd81

This command fails. I think the error message is something like "the
remote end hung up unexpectedly", on Debian Buster. A more useful
message with more recent version of git seems to be "couldn't find
remote ref".

If we allow git to communicate with the protocol v2, then the shallow
clone works. But git on buster still use v1 by default. Force it to
use v2.

This needs a git-cache-proxy version that can allow to switch to v2 of
the protocol. A bug is open upstream to track this change:
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1040476

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>