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We should add a "git gc --auto" after "git clone" due to commit graph

Don't have time to patch this now, but thought I'd send a note / RFC
about this.

Now that we have the commit graph it's nice to be able to set
e.g. core.commitGraph=true & gc.writeCommitGraph=true in ~/.gitconfig or
/etc/gitconfig to apply them to all repos.

But when I clone e.g. linux.git stuff like 'tag --contains' will be slow
until whenever my first "gc" kicks in, which may be quite some time if
I'm just using it passively.

So we should make "git gc --auto" be run on clone, and change the
need_to_gc() / cmd_gc() behavior so that we detect that the
gc.writeCommitGraph=true setting is on, but we have no commit graph, and
then just generate that without doing a full repack.

As an aside such more granular "gc" would be nice for e.g. pack-refs
too. It's possible for us to just have one pack, but to have 100k loose

It might also be good to have some gc.autoDetachOnClone option and have
it false by default, so we don't have a race condition where "clone
linux && git -C linux tag --contains" is slow because the graph hasn't
been generated yet, and generating the graph initially doesn't take that
long compared to the time to clone a large repo (and on a small one it
won't matter either way).

I was going to say "also for midx", but of course after clone we have
just one pack, so I can't imagine us needing this. But I can see us
having other such optional side-indexes in the future generated by gc,
and they'd also benefit from this.