I read 'program name' as referring to 'Mercurial', not 'hg'. Perhaps Tshepang did also. Read that way, it is not right. Reading it the intended way is not so obvious to one who has never typed 'hg' on a command line. It would be impossible for one who does not even know that hg is the executable name.

Given that the intended audience is not limited to experienced hg users, I suggest changing "(commonly abbreviated hg, after the program name)" to the much clearer and explicit "The Mercurial executable is named 'hg', and 'hg' is often used to refer to Mercurial itself." (This is just a slight rewording of what Éric said is meant to be said.) 

I agree that we should not discuss the origin of 'hg'. The chemical symbol is 'Hg', whereas 'hg' (normally) abbreviates 'hectogram'. My suggestion simply states two facts that beginning cpython developers need to know.