changegroup: note during bundle apply if the repo was empty

An upcoming change for exchanging treemanifest data will need to
update the repository capabilities, which we should only do if the
repository was empty before we started applying this changegroup. In
the future we will probably need a strategy for upgrading to
treemanifest in requires during a pull (I'm assuming at some point
we'll make it possible to have a flag day to enable treemanifests on
an existing history.)

mercurial/changegroup.py

@@ -312,6 +312,7 b' class cg1unpacker(object):'
         - number of heads stays the same: 1
         """
         repo = repo.unfiltered()
+        wasempty = (len(repo.changelog) == 0)
         def csmap(x):
             repo.ui.debug("add changeset %s\n" % short(x))
             return len(cl)
@@ -386,7 +387,7 b' class cg1unpacker(object):'
             self.callback = None
             pr = prog(_('files'), efiles)
             newrevs, newfiles = _addchangegroupfiles(
-                repo, self, revmap, trp, pr, needfiles)
+                repo, self, revmap, trp, pr, needfiles, wasempty)
             revisions += newrevs
             files += newfiles
 
@@ -903,7 +904,7 b' def changegroup(repo, basenodes, source)'
     # to avoid a race we use changegroupsubset() (issue1320)
     return changegroupsubset(repo, basenodes, repo.heads(), source)
 
-def _addchangegroupfiles(repo, source, revmap, trp, pr, needfiles):
+def _addchangegroupfiles(repo, source, revmap, trp, pr, needfiles, wasempty):
     revisions = 0
     files = 0
     while True: