.arcconfig

util: implement `writelines()` on atomictempfile

With typehints on the vfs objects, pytype will flag this:

    FAILED: /mnt/c/Users/Matt/hg/.pytype/pyi/mercurial/patch.pyi
    /usr/bin/python3.8 -m pytype.single
        --imports_info /mnt/c/Users/Matt/hg/.pytype/imports/mercurial.patch.imports
        --module-name mercurial.patch -V 3.7
        -o /mnt/c/Users/Matt/hg/.pytype/pyi/mercurial/patch.pyi
        --analyze-annotated --nofail --quick
        /mnt/c/Users/Matt/hg/mercurial/patch.py
    File "/mnt/c/Users/Matt/hg/mercurial/patch.py", line 535, in writerej:
        No attribute 'writelines' on mercurial.util.atomictempfile [attribute-error]
      In Union[
          mercurial.util.atomictempfile,
          mercurial.vfs.checkambigatclosing,
          mercurial.vfs.delayclosedfile,
          mercurial.windows.fdproxy,
          mercurial.windows.mixedfilemodewrapper
      ]

It's not a real problem there (atomictempfile is only created by passing
different args), but it's reasonable for this to implement the function and
behave like a normal file.  There are other functions missing that can be added
if/when needed.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}