obsolete: make _computeobsoleteset much faster

This patch makes _computeobsoleteset much faster by looping
over the draft and secrets as opposed to looping over the
successors.
This works because "number of draft and secret" is typically
way smaller(<100) than the number of successor in the repo (~90k in
my checkout of core mercurial as of today). And also because
it is very fast to compute "not public()".

I timed the code with the following setup:
"""
from mercurial import hg, ui, obsolete
ui = ui.ui()
repo = hg.repository(ui, "~/hg")
l = repo.obsstore.successors # This caches the result
"""

With about 90k successors.

k=obsolete._computeobsoleteset(repo) before this patch:
10 loops, best of 3: 33.9 ms per loop

k=obsolete._computeobsoleteset(repo) after this patch:
10000 loops, best of 3: 83.3 µs per loop

mercurial/obsolete.py

@@ -1119,12 +1119,11 b' def clearobscaches(repo):'
 def _computeobsoleteset(repo):
     """the set of obsolete revisions"""
     obs = set()
-    getrev = repo.changelog.nodemap.get
-    getphase = repo._phasecache.phase
-    for n in repo.obsstore.successors:
-        rev = getrev(n)
-        if rev is not None and getphase(repo, rev):
-            obs.add(rev)
+    getnode = repo.changelog.node
+    notpublic = repo.revs("not public()")
+    for r in notpublic:
+        if getnode(r) in repo.obsstore.successors:
+            obs.add(r)
     return obs
 
 @cachefor('unstable')