Use case

A laptop with a relatively small hard drive has copies of a subset of all annexed files. When annexed files are changed externally and git annex sync is run on the laptop, the stale local copies are invalidated and their symlinks break. How can I automatically fetch the updated versions of these previously locally-cached files?

Because I only want a subset of files, I can't do

git annex add --not --in here --and --in superset.

Because files may be renamed, the automatically getting files on checkout solution, by making dir specify the subset, will require manually and redundantly tracking renames.

Simple ( ) feature addition to git-annex to support this

When locally-cached files are invalidated by git-annex sync, git-annex could notify the user, and give them the option to git-annex get the invalidated files. Bonus points if the mechanism allows this to be done at any point in the future, not just when running git-annex sync. The idea is that git-annex could track which files, previously cached locally, have been invalidated unintentionally by syncs, and treat them differently from files, previously cached locally, that have been intentionally dropped using git-annex drop or git-annex move.

More generally

The ability to specify a collection of files to always cache locally (something like a numcopies.here=1), which is robust to renames, would work. The "robust to renames" part seems tricky in git: whereas svn attaches properties to files, and so properties are propagated by svn mv, I believe git attributes are only specified by patterns in .gitattributes files.

Related questions / possible approaches

Other forum posts mention git subtree and sparse git checkouts, but I'm not familiar with these features and from reading those questions it's unclear if those approaches will work for me. Does anyone more familiar see how to adapt one of those features to my use case?