test-grep: add tests for --follow with/without --diff and/or paths
This tests the behavior of cmdutil.walkchangerevs().
I'm going to rewrite cmdutil.walkchangerevs() to leverage the "log -f"
logic, but the code coverage looked quite small. And the history traversal
of "grep -f" goes wrong in various ways.
{
// 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)
}