Keep worktree relative to GIT_DIR

core.worktree can be set relatively to GIT_DIR, thereby preventing the
hard-coding of the home directory path, which should make a vcsh setup
more portable.

On the other hand, this means that the vcsh repo.d is now invariably
linked to its position, relative to the base directory. Apply this patch
only if you think it to be less likely that people change
~/.config/vcsh/repo.d to something else, than they would rsync/tar/mv
a whole tree to a different $HOME.

Paths are resolved using Git itself, for maximum portability.

In addition to making sure the new method works on new repositories,
I checked that

  - "old" repositories can be used with the new code just fine;
  - upgrading works.

Signed-off-by: martin f. krafft <madduck@madduck.net>