It seems that what you are trying to do is
git push heroku master
or, meaning the same if you are currently on branchname
git push heroku branchname:master
From manpage:
git push [repository [refspec...]]
refspec...
Specify what destination ref to update with what source object. The format of a <refspec> parameter is an optional plus +, followed by the source object <src>, followed by a colon :, followed by the destination ref <dst>.