discovery: prevent recomputing info about server and outgoing changesets

We already iterate over the outgoing.missing above and lookup repo for them. So
let's reuse info calculated at that time instead of recomputing that again.

Also we calculate the set of remotebranches by doing set(remotemap), so let's
reuse that again.

Upcoming patches will clean things a bit more.

Differential Revision: https://phab.mercurial-scm.org/D6144

mercurial/discovery.py

@@ -201,19 +201,24 b' def _headssummary(pushop):'
     outgoing = pushop.outgoing
     cl = repo.changelog
     headssum = {}
+    missingctx = set()
     # A. Create set of branches involved in the push.
-    branches = set(repo[n].branch() for n in outgoing.missing)
+    branches = set()
+    for n in outgoing.missing:
+        ctx = repo[n]
+        missingctx.add(ctx)
+        branches.add(ctx.branch())
+    nbranches = branches.copy()
 
     with remote.commandexecutor() as e:
         remotemap = e.callcommand('branchmap', {}).result()
 
-    newbranches = branches - set(remotemap)
+    remotebranches = set(remotemap)
+    newbranches = branches - remotebranches
     branches.difference_update(newbranches)
 
     # A. register remote heads
-    remotebranches = set()
     for branch, heads in remotemap.iteritems():
-        remotebranches.add(branch)
         known = []
         unsynced = []
         knownnode = cl.hasnode # do not use nodemap until it is filtered
@@ -224,16 +229,12 b' def _headssummary(pushop):'
                 unsynced.append(h)
         headssum[branch] = (known, list(known), unsynced)
     # B. add new branch data
-    missingctx = list(repo[n] for n in outgoing.missing)
-    touchedbranches = set()
-    for ctx in missingctx:
-        branch = ctx.branch()
-        touchedbranches.add(branch)
+    for branch in nbranches:
         if branch not in headssum:
             headssum[branch] = (None, [], [])
 
     # C drop data about untouched branches:
-    for branch in remotebranches - touchedbranches:
+    for branch in remotebranches - nbranches:
         del headssum[branch]
 
     # D. Update newmap with outgoing changes.