annotate: discard refcount of discarded annotation for memory efficiency

Before this patch, refcount (managed in "needed") of the annotation
result is kept as 1, even if corresponding annotation result is
discarded from "hist", because it isn't decreased and discarded.

In the history tree including merging revision, the most recent common
ancestor of merged revisions is scanned twice. Refcount of such
ancestor never becomes 0, because refcount is started from 1 at the
second scanning.

This prevents annotation results of merging revision in "hist" from
being discarded, and decreases memory efficiency.

This patch discards refcount of the annotation result, when the
corresponding annotation is discarded from "hist".

mercurial/context.py

@@ -713,6 +713,7 b' class filectx(object):'
                         curr = pair(hist[p], curr)
                     if needed[p] == 1:
                         del hist[p]
+                        del needed[p]
                     else:
                         needed[p] -= 1