v6: When annex.largefiles is not configured for a file, running git add or
git commit, or otherwise using git to stage a file will add it to the annex
if the file was in the annex before, and to git otherwise. This is to avoid
accidental conversion.
Note that git-annex add's behavior has not changed, for reasons explained
in the added comment.
Performance: No added overhead when annex.largefiles is configured.
When not configured, there is an added call to catObjectMetaData,
which involves a round trip through git cat-file --batch.
However, the earlier catKeyFile primes the cache for it.
This commit was supported by the NSF-funded DataLad project.
