[log] simplify loading details around selected commit

No need to implement some AroundProvider: we can take data right from
the table model, by row index.

DataGetter#getCommitData has to be Nullable now, because there can be
"empty" rows holding no commits.

VcsLogFilterer can't use new getCommitData, because there is no table
model to supply: it actually gets created from the filterer. However,
we expect details to be in the cache, so there is no need to load
around anyway.

(cherry picked from commit 2004462f86f397b7bed95bec4927b1fe87472377)