changegroup: store old heads as a set

Previously, the "oldheads" variable was a list. On a repository at
Mozilla with 46,492 heads, profiling revealed that list membership
testing was dominating execution time of applying small changegroups.

This patch converts the list of old heads to a set. This makes
membership testing significantly faster. On the aforementioned
repository with 46,492 heads:

$ hg unbundle <file with 1 changeset>
before: 18.535s wall
after:   1.303s

Consumers of this variable only check for truthiness (`if oldheads`),
length (`len(oldheads)`), and (most importantly) item membership
(`h not in oldheads` - which occurs twice). So, the change to a set
should be safe and suitable for stable.

The practical effect of this change is that changegroup application
and related operations (like `hg push`) no longer exhibit an O(n^2)
CPU explosion as the number of heads grows.

mercurial/changegroup.py

@@ -293,7 +293,7 b' class cg1unpacker(object):'
                 # will not see an inconsistent view
                 cl = repo.changelog
                 cl.delayupdate(tr)
-                oldheads = cl.heads()
+                oldheads = set(cl.heads())
 
                 trp = weakref.proxy(tr)
                 # pull off the changeset group