configure: remove --with-git= option

The scenario for which --with-git= was introduced was to use a SOCKS proxy
such as tsocks.  However, this was back in 2017 when QEMU's submodules
used the git:// protocol, and it is not as important when using the
"smart HTTP" backend; for example, neither "meson subprojects download"
nor scripts/checkpatch.pl obey the GIT environment variable.

So remove the knob, but test for the presence of git in the configure and
git-submodule.sh scripts, and suggest using --with-git-submodules=validate
+ a manual invocation of git-submodule.sh when git does not work.  Hopefully
in the future the GIT environment variable will be supported by Meson.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/ci/org.centos/stream/8/x86_64/configure

@@ -29,14 +29,12 @@
 --extra-cflags="-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection" \
 --with-suffix="qemu-kvm" \
 --firmwarepath=/usr/share/qemu-firmware \
---with-git=meson \
 --with-git-submodules=update \
 --target-list="x86_64-softmmu" \
 --block-drv-rw-whitelist="qcow2,raw,file,host_device,nbd,iscsi,rbd,blkdebug,luks,null-co,nvme,copy-on-read,throttle,gluster" \
 --audio-drv-list="" \
 --block-drv-ro-whitelist="vmdk,vhdx,vpc,https,ssh" \
 --with-coroutine=ucontext \
---with-git=git \
 --tls-priority=@QEMU,SYSTEM \
 --disable-attr \
 --disable-auth-pam \