restage pointer file after unlock

restage pointer file after unlock

This avoids a later git status or similar taking a long time to run
as it runs git-annex smudge once per file. While v9 repositories do
avoid that taking long when the files are small, large files can still
make git status take a very long time.

This does make unlock slower, because now git-annex smudge is being run
once per file unlocked. However, the next commit should speed that up in
many cases.

Sponsored-by: Boyd Stephen Smith Jr. on Patreon