[PATCH 0 of 2 STABLE] annotation: fix problems in the tree merging a revision and its ancestor

[FUJIWARA Katsunori]

This patch series fixes issue3841.

You'll notice one more issue in "filectx.annotation" that annotation
result for the recent common ancestor of merged revisions is kept in
"hist", even if it isn't referred any more.

I'm planning to post it separately as improvement of memory
efficiency, because it doesn't affect result of annotation.