backout: correct commit status of no changes made (BC) (issue4190)

If backout generated no changes to commit, it showed wrong status, "changeset
<target> backs out changeset <target>", and raised TypeError with -v option.

This changes the return code to 1, which is the same as "hg commit" and
"hg rebase".

mercurial/commands.py

@@ -416,7 +416,8 b' def backout(ui, repo, node=None, rev=Non'
 
     See :hg:`help dates` for a list of formats valid for -d/--date.
 
-    Returns 0 on success, 1 if there are unresolved files.
+    Returns 0 on success, 1 if nothing to backout or there are unresolved
+    files.
     '''
     if rev and node:
         raise util.Abort(_("please specify just one revision"))
@@ -495,6 +496,9 b' def backout(ui, repo, node=None, rev=Non'
             return repo.commit(message, opts.get('user'), opts.get('date'),
                                match, editor=e)
         newnode = cmdutil.commit(ui, repo, commitfunc, [], opts)
+        if not newnode:
+            ui.status(_("nothing changed\n"))
+            return 1
         cmdutil.commitstatus(repo, newnode, branch, bheads)
 
         def nice(node):

tests/test-backout.t

@@ -409,6 +409,22 b' on branch1, so no file1 and file2:'
 
   $ cd ..
 
+backout of empty changeset (issue4190)
+
+  $ hg init emptycommit
+  $ cd emptycommit
+
+  $ touch file1
+  $ hg ci -Aqm file1
+  $ hg branch -q branch1
+  $ hg ci -qm branch1
+  $ hg backout -v 1
+  resolving manifests
+  nothing changed
+  [1]
+
+  $ cd ..
+
 
 Test usage of `hg resolve` in case of conflict
 (issue4163)