json: show_contexts gives every function and class the whole file's contexts #2374

coverage json --show-contexts adds a contexts dict to every entry under functions and classes, but each of those gets the contexts for the entire file, not just the lines in that function or class.

m.py:

def f():
    return 1

def g():
    return 2

f()
g()

coverage run --context=t1 m.py
coverage json --show-contexts -o out.json

files["m.py"]["functions"]["f"]["contexts"] has lines 1, 2, 4, 5, 7 and 8. I'd expect only line 2.

In jsonreport.py, make_region_data calls contexts_by_lineno() for the whole file and uses the result as is. It also runs that query once per region.