add a small cache of the most recently accessed item from the git-annex branch

This will speed up typical cases like git-annex get, which currently
has to read the location log once, then read it a second time in order to
add a line to it. Since these reads now involve more than just reading
in a file, it seemed good to add a cache layer.

Only the most recent thing needs to be cached, because git-annex has
good locality; it operates on one file at a time, and only cares
about one item from the branch per file.

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