merge: handle no file parent in backwards merge (issue2364)

mercurial/merge.py

@@ -276,8 +276,11 def applyupdates(repo, action, wctx, mct
             fcl = wctx[f]
             fco = mctx[f2]
             if mctx == actx: # backwards, use working dir parent as ancestor
+                if fcl.parents():
                 fca = fcl.parents()[0]
             else:
+                    fca = repo.filectx(f, fileid=nullrev)
+            else:
                 fca = fcl.ancestor(fco, actx)
             if not fca:
                 fca = repo.filectx(f, fileid=nullrev)

tests/test-merge1.t

@@ -166,3 +166,11 merge of b expected
   +This is file b33
   $ hg status
   M b
+
+Test for issue2364
+
+  $ hg up -qC .
+  $ hg rm b
+  $ hg ci -md
+  $ hg revert -r -2 b
+  $ hg up -q -- -2