merge: short-circuit search for merge into empty repo

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merge: short-circuit search for merge into empty repo

We should have 3 cases for merge:

- - we have no changesets
- - we have less than half the changesets
- - we have more than half the changesets

For no changesets, we can immediately tell that we need everything.
This happens when we initially branch from a remote repo, so we simply shortcircuit the search and grab everything from the root

When we're actually tracking a project, we should generally have most
of the changesets, so the current search algorithm should minimize
searching.

It should rarely occur that upstreams gets far ahead of us, in which
case, we suffer a longer search.

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mercurial/hg.py

@@ -525,15 +525,21 @@ class localrepository:
         return nl
 
     def getchangegroup(self, remote):
-        tip = remote.branches([])[0]
-        self.ui.debug("remote tip branch is %s:%s\n" %
-                      (short(tip[0]), short(tip[1])))
         m = self.changelog.nodemap
-        unknown = [tip]
         search = []
         fetch = []
         seen = {}
         seenbranch = {}
+        tip = remote.branches([])[0]
+        self.ui.debug("remote tip branch is %s:%s\n" %
+                      (short(tip[0]), short(tip[1])))
+
+        # if we have an empty repo, fetch everything
+        if self.changelog.tip() == nullid:
+            return remote.changegroup([nullid])
+
+        # otherwise, assume we're closer to the tip than the root
+        unknown = [tip]
 
         if tip[0] in m:
             self.ui.note("nothing to do!\n")