Manage .gitignore per gitjoin.txt directory

Each gitjoin.txt now updates only its sibling .gitignore, listing its
repos by basename. Stale managed sections in .gitignore files whose
directory has no gitjoin.txt are pruned on sync, which both fixes the
migration from the old root-level layout and makes runs idempotent
regardless of which subdirectory gitjoin is invoked from.

readme: Why not use Git Submodules instead?