Before:
  Save g:ale_warn_about_trailing_whitespace

  let g:ale_warn_about_trailing_whitespace = 1

  runtime ale_linters/gitcommit/gitlint.vim

After:
  Restore

  unlet! b:ale_warn_about_trailing_whitespace

  call ale#linter#Reset()

Execute(The gitlint handler should handle basic warnings and syntax errors):
  AssertEqual
  \ [
  \   {
  \     'lnum': 1,
  \     'type': 'E',
  \     'text': 'Body message is missing',
  \     'code': 'B6',
  \   },
  \   {
  \     'lnum': 2,
  \     'type': 'E',
  \     'text': 'Second line is not empty: "to send to upstream"',
  \     'code': 'B4',
  \   },
  \   {
  \     'lnum': 3,
  \     'type': 'E',
  \     'text': 'Body message is too short (19<20): "to send to upstream"',
  \     'code': 'B5',
  \   },
  \   {
  \     'lnum': 8,
  \     'type': 'E',
  \     'text': 'Title exceeds max length (92>72): "some very long commit subject line where the author can''t wait to explain what he just fixed"',
  \     'code': 'T1',
  \   },
  \ ],
  \ ale_linters#gitcommit#gitlint#Handle(1, [
  \   '1: B6 Body message is missing',
  \   '2: B4 Second line is not empty: "to send to upstream"',
  \   '3: B5 Body message is too short (19<20): "to send to upstream"',
  \   '8: T1 Title exceeds max length (92>72): "some very long commit subject line where the author can''t wait to explain what he just fixed"'
  \ ])

Execute(Disabling trailing whitespace warnings should work):
  AssertEqual
  \ [
  \   {
  \     'lnum': 8,
  \     'type': 'E',
  \     'text': 'Trailing whitespace',
  \     'code': 'T2',
  \   },
  \ ],
  \ ale_linters#gitcommit#gitlint#Handle(bufnr(''), [
  \   '8: T2 Trailing whitespace',
  \])

  let b:ale_warn_about_trailing_whitespace = 0

  AssertEqual
  \ [],
  \ ale_linters#gitcommit#gitlint#Handle(bufnr(''), [
  \   '8: T2 Trailing whitespace',
  \ ])