alias: do not crash when aliased command has no usage help text

mercurial/dispatch.py

@@ -196,7 +196,12 b' class cmdalias(object):'
         cmd = args.pop(0)
 
         try:
-            self.fn, self.opts, self.help = cmdutil.findcmd(cmd, cmdtable, False)[1]
+            tableentry = cmdutil.findcmd(cmd, cmdtable, False)[1]
+            if len(tableentry) > 2:
+                self.fn, self.opts, self.help = tableentry
+            else:
+                self.fn, self.opts = tableentry
+
             self.args = aliasargs(self.fn) + args
             if cmd not in commands.norepo.split(' '):
                 self.norepo = False

tests/test-alias

@@ -12,6 +12,7 b' mylog = log'
 lognull = log -r null
 shortlog = log --template '{rev} {node|short} | {date|isodate}\n'
 dln = lognull --debug
+nousage = rollback
 
 [defaults]
 mylog = -q
@@ -35,6 +36,10 b" echo '% no definition'"
 hg nodef
 
 cd alias
+
+echo '% no usage'
+hg nousage
+
 echo foo > foo
 hg ci -Amfoo
 

tests/test-alias.out

@@ -7,6 +7,8 b" alias 'ambiguous' resolves to ambiguous "
 alias 'recursive' resolves to unknown command 'recursive'
 % no definition
 no definition for alias 'nodefinition'
+% no usage
+no rollback information available
 adding foo
 % with opts
 C foo