got an older qemu checkout? please run 'git submodule sync'

[Peter Maydell]

Hi; if you have an QEMU checkout that you created more than a couple
of years ago and are still using, you should run:
   git submodule sync
in it to refresh git's idea of where all our submodules live.

This is necessary because git submodules are (as usual) broken;
in particular, when git first checks out the submodule it stores
the URL it used to do that inside the .git directory in your checkout.
Even if we update .gitmodules later to point somewhere else, git
doesn't automatically update the .git metadata. This means that if
that older URL is no longer working and the submodule needs to be
updated, then git will produce an error, for example:

Fetching submodule tests/fp/berkeley-testfloat-3
fatal: unable to connect to github.com:
github.com[0: 140.82.121.3]: errno=Connection timed out

One such change is about to land in upstream QEMU. If you run
'git submodule sync' now then this will preemptively avoid the
problem where you find that trying to 'git pull' or
'git submodule update' doesn't work.

thanks
-- PMM