calcGitLink turns out to need it to be absolute, and it normally is,
but not if it's read from a .git file in a submodule, or perhaps from
GIT_DIR.
I should look into dropping this invariant.
Now that this is handled correctly, git-annex can be used in git submodules.
Also, fixed infelicity where Git.CurrentRepo and Git.Config.updateLocation
were both dealing with core.worktree. Now updateLocation handles it for
Local as well as for LocalUnknown repos.
Avoid more expensive code path when no core.worktree is configured.
Don't change worktree when reading config if one is already set.
This could happen if GIT_CORE_WORKTREE is set, and the repo also has
core.worktree, and the config is reread. Now GIT_CORE_WORKTREE will
prevail.
The environment needs to override git-config. Changed when git config is
read, and avoid rereading it once it's been read.
chdir for both worktree settings.
