admin: resolve inboxes to absolute paths for index
Some of my ancient v1-only scripts called public-inbox-index
to operate on GIT_DIR:
GIT_DIR=/path/to/foo.git public-inbox-index
This change ensures they keep working, otherwise "." will be
passed to the --git-dir= switch of git(1) because that's the
default directory if no inboxes are specified on the
command-line.