add experimental git worktree support

This provides initial support for using git worktrees internally
instead of our own ad-hoc symlink tree.  It's been lightly tested
which is why it's not currently exposed via --help.

When people opt-in to worktrees in an existing repo client checkout,
no projects are migrated.  Instead, only new projects will use the
worktree method.  This allows for limited testing/opting in without
having to completely blow things away or get a second checkout.

Bug: https://crbug.com/gerrit/11486
Change-Id: Ic3ff891b30940a6ba497b406b2a387e0a8517ed8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254075
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>