annotate: make pair take all parents to pair against

In upcoming patches we'll need to be aware of all parents at the same time.

This also exposes a potential bug: if a line can be annotated with both parents
of a merge commit, it'll always be annotated with p2, not p1. I'm not sure if
that's what we want, but at least the code makes it clear now.

mercurial/context.py

@@ -1044,8 +1044,8 b' class basefilectx(object):'
             if ready:
                 visit.pop()
                 curr = decorate(f.data(), f)
+                curr = _annotatepair([hist[p] for p in pl], curr, diffopts)
                 for p in pl:
-                    curr = _annotatepair(hist[p], curr, diffopts)
                     if needed[p] == 1:
                         del hist[p]
                         del needed[p]
@@ -1073,8 +1073,12 b' class basefilectx(object):'
             c = visit.pop(max(visit))
             yield c
 
-def _annotatepair(parent, child, diffopts):
-    blocks = mdiff.allblocks(parent[1], child[1], opts=diffopts)
+def _annotatepair(parents, child, diffopts):
+    pblocks = [(parent, mdiff.allblocks(parent[1], child[1], opts=diffopts))
+               for parent in parents]
+    # Mercurial currently prefers p2 over p1 for annotate.
+    # TODO: change this?
+    for parent, blocks in pblocks:
     for (a1, a2, b1, b2), t in blocks:
         # Changed blocks ('!') or blocks made only of blank lines ('~')
         # belong to the child.