I should also note that git seems to be able to launch the editor.

The git update appears to be involved as that's when it breaks:
commit 534b85903cc93f3b9fbeff9b43747ba34fad7320
Date:   Thu Mar 31 01:36:44 2016 +0000

    Make git 2.7.4 the default version

Printf debugging at subprocess2.communicate seems to indicate this difference:

Old Git:
env 'C:/Users/robliao/AppData/Local/GitNPP/notepad++.exe' -multiInst -notabbar -nosession -noPlugin [Path to CL Description]

New Git:
'C:/Users/robliao/AppData/Local/GitNPP/notepad++.exe' -multiInst -notabbar -nosession -noPlugin [Path to CL Description]

Note the missing env.

Looks like the git update has changed some assumptions here:

def RunEditor(content, git, git_editor=None):
  """Opens up the default editor in the system to get the CL description."""
  [...]
  try:
    editor = GetEditor(git, git_editor=git_editor)
    if not editor:
      return None
    cmd = '%s %s' % (editor, filename)
    # <----------- Line of interest after this point
    if sys.platform == 'win32' and os.environ.get('TERM') == 'msys':
      # Msysgit requires the usage of 'env' to be present.
      cmd = 'env ' + cmd
    [...]

env --> D:\depot_tools\git-2.7.4-64_bin\usr\bin\env.exe
Looks like env was doing some rudimentary quote handling.

In git 1.9.5, the quotes were needed to get the 'git commit' command to fire up an editor opened to the commit message, otherwise you'd get a mysterious '$@' doesn't exist.

In git 2.7.4, launching without quotes appears to be fixed.

Removing the quotes from my core.editor setting fixes git cl and git commit is usable on 2.7.4.