tests/heredoctest.py

editor: file created for diff action should have .diff suffix

This is a follow-up to https://phab.mercurial-scm.org/D464 (6e6452bc441d) that
introduced the new file extension behavior. It erroneously changed `.diff` to
`.diff.hg.txt`.

Test Plan:
Verified `make tests` passes, particularly `test-editor-filename.t`.

Differential Revision: https://phab.mercurial-scm.org/D607

from __future__ import absolute_import, print_function

import sys

globalvars = {}
lines = sys.stdin.readlines()
while lines:
    l = lines.pop(0)
    if l.startswith('SALT'):
        print(l[:-1])
    elif l.startswith('>>> '):
        snippet = l[4:]
        while lines and lines[0].startswith('... '):
            l = lines.pop(0)
            snippet += l[4:]
        c = compile(snippet, '<heredoc>', 'single')
        try:
            exec(c, globalvars)
        except Exception as inst:
            print(repr(inst))