annotate: reuse already calculated annotation

Before this patch, annotation is re-calculated even if it is already
calculated. This may cause unexpected annotation, because already
cleared "pcache" ("pcache[f] = []") prevents from scanning ancestors.

This patch reuses already calculated annotation if it is available.

In fact, "reusable" situation should be seen only on legacy
repositories in which a filelog include the merging between the
revision and its ancestor, because:

  - tree is scanned in depth-first

    without such merging, annotation result should be released soon

  - recent Mercurial doesn't allow such merging

    changelog and manifest can include such merging someway, but
    filelogs can't, because "localrepository._filecommit()" converts
    such merging request to linear history.

mercurial/context.py

@@ -701,9 +701,14 b' class filectx(object):'
                     needed[p] = needed.get(p, 0) + 1
             if ready:
                 visit.pop()
-                curr = decorate(f.data(), f)
+                reusable = f in hist
+                if reusable:
+                    curr = hist[f]
+                else:
+                    curr = decorate(f.data(), f)
                 for p in pl:
-                    curr = pair(hist[p], curr)
+                    if not reusable:
+                        curr = pair(hist[p], curr)
                     if needed[p] == 1:
                         del hist[p]
                     else: