This will mostly just avoid a DB lookup, so things get marginally
faster. But in cases where there are many files using the same key, it
can be a more significant speedup.
Added overhead is one MVar lookup per call, which should be small
enough, since this happens after transferring or ingesting a file,
which is always a lot more work than that. It would be nice, though,
to move getGitConfig to AnnexRead, which there is an open todo about.
