handle deleted files when getting unstaged changes

If one of the changes that hasn't been staged involves deleting a file,
blob_from_path_and_stat() throws an OSError. Trap the error and treat
the deleted file as a change by emitting the name of the file.

This bug was reported downstream against reno as
https://bugs.launchpad.net/reno/+bug/1655719

Signed-off-by: Doug Hellmann <doug@doughellmann.com>