Fix and test 'hg backout' without or with too many revisions.

mercurial/commands.py

@@ -169,6 +169,9 b' def backout(ui, repo, node=None, rev=Non'
     if not rev:
         rev = node
 
+    if not rev:
+        raise util.Abort(_("please specify a revision to backout"))
+
     cmdutil.bail_if_changed(repo)
     op1, op2 = repo.dirstate.parents()
     if op2 != nullid:

tests/test-backout

@@ -2,9 +2,14 b''
 
 HGMERGE=true; export HGMERGE
 
-echo '# basic operation'
 hg init basic
 cd basic
+
+echo '# should complain'
+hg backout
+hg backout -r 0 0
+
+echo '# basic operation'
 echo a > a
 hg commit -d '0 0' -A -m a
 echo b >> a

tests/test-backout.out

@@ -1,3 +1,6 b''
+# should complain
+abort: please specify a revision to backout
+abort: please specify just one revision
 # basic operation
 adding a
 reverting a