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

mercurial/merge.py

@@ -276,7 +276,10 b' def applyupdates(repo, action, wctx, mct'
             fcl = wctx[f]
             fco = mctx[f2]
             if mctx == actx: # backwards, use working dir parent as ancestor
-                fca = fcl.parents()[0]
+                if fcl.parents():
+                    fca = fcl.parents()[0]
+                else:
+                    fca = repo.filectx(f, fileid=nullrev)
             else:
                 fca = fcl.ancestor(fco, actx)
             if not fca:

tests/test-merge1.t

@@ -166,3 +166,11 b' 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