git-hgext: adjust to the lack of `changelog.heads` method
We don't have a `heads` method returning nodeid, but this is very easy to get
the same result.
This was flagged by pytype.
We can note that the fact this code did not break is probably a good sign that
it is dead code.
However this is a question outside of the scop of this series.
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}