TODO

• git-sym fix -- also fix broken links from moved cache, and missing links in GIT_SYM_DIR
• Try .gitattributes instead of .gitignore, to avoid problems with git clean.
• Add git-submodule support, to run git-sym update automatically.
• Re-create symlink only when necessary?
• A 'force' mode to re-create symlinks, in case we point to an old cache.
• A 'refresh' sub-command, to re-run all makefile rules (though that might be a no-op) and to re-create symlinks.