emitrevision: consider ancestors revision to emit as available base

This should make more delta base valid. This notably affects:

* case where we skipped some parent with empty delta to directly delta against
  an ancestors
* case where an intermediate snapshots is stored.

This change means we could sent largish intermediate snapshots over the wire.
However this is actually a sub goal here. Sending snapshots over the wire means
the client have a high odd of simply storing the pre-computed delta instead of
doing a lengthy process that will… end up doing the same intermediate snapshot.

In addition the overall size of snapshot (or any level) is "only" some or the
overall delta size. (0.17% for my mercurial clone, 20% for my clone of Mozilla
try). So Sending them other the wire is unlikely to change large impact on the
bandwidth used.

If we decide that minimising the bandwidth is an explicit goal, we should
introduce new logic to filter-out snapshot as delta. The current code has no
notion explicite of snapshot so far, they just tended to fall into the wobbly
filtering options.

In some cases, this patch can yield large improvement to the bundling time:

    ### data-env-vars.name      = mozilla-try-2019-02-18-zstd-sparse-revlog
      # benchmark.name          = perf-bundle
      # benchmark.variants.revs = last-100000
    before: 68.787066 seconds
    after:  47.552677 seconds (-30.87%)

That translate to large improvement to the pull time :

    ### data-env-vars.name           = mozilla-try-2019-02-18-zstd-sparse-revlog
      # benchmark.name               = pull
      # benchmark.variants.issue6528 = disabled
      # benchmark.variants.revs      = last-100000
    before: 142.186625 seconds
    after:   75.897745 seconds  (-46.62%)

No significant negative impact have been observed.

mercurial/utils/storageutil.py

@@ -374,6 +374,7 b' def emitrevisions('
 
     fnode = store.node
     frev = store.rev
+    parents = store.parentrevs
 
     if nodesorder == b'nodes':
         revs = [frev(n) for n in nodes]
@@ -386,30 +387,28 b' def emitrevisions('
     prevrev = None
 
     if deltamode == repository.CG_DELTAMODE_PREV or assumehaveparentrevisions:
-        prevrev = store.parentrevs(revs[0])[0]
+        prevrev = parents(revs[0])[0]
 
-    # Set of revs available to delta against.
+    # Sets of revs available to delta against.
+    emitted = set()
     available = set()
-    parents = []
+    if assumehaveparentrevisions:
+        common_heads = set(p for r in revs for p in parents(r))
+        common_heads.difference_update(revs)
+        available = store.ancestors(common_heads, inclusive=True)
 
     def is_usable_base(rev):
         """Is a delta against this revision usable over the wire"""
         if rev == nullrev:
             return False
-        # Base revision was already emitted in this group.
-        if rev in available:
-            return True
-        # Base revision is a parent that hasn't been emitted already.
-        if assumehaveparentrevisions and rev in parents:
-            return True
-        return False
+        return rev in emitted or rev in available
 
     for rev in revs:
         if rev == nullrev:
             continue
 
         node = fnode(rev)
-        parents[:] = p1rev, p2rev = store.parentrevs(rev)
+        p1rev, p2rev = parents(rev)
 
         if deltaparentfn:
             deltaparentrev = deltaparentfn(rev)
@@ -481,7 +480,7 b' def emitrevisions('
                 baserev == nullrev and deltamode != repository.CG_DELTAMODE_PREV
             ):
                 revision = store.rawdata(node)
-                available.add(rev)
+                emitted.add(rev)
             else:
                 if revdifffn:
                     delta = revdifffn(baserev, rev)
@@ -490,7 +489,7 b' def emitrevisions('
                         store.rawdata(baserev), store.rawdata(rev)
                     )
 
-                available.add(rev)
+                emitted.add(rev)
 
         serialized_sidedata = None
         sidedata_flags = (0, 0)