Fix #2399 - Do not check buffers used for displaying diffs

autoload/ale.vim

@@ -43,6 +43,11 @@ function! ale#ShouldDoNothing(buffer) abort
         return 1
     endif
 
+    " Do nothing for diff buffers.
+    if getbufvar(a:buffer, '&diff')
+        return 1
+    endif
+
     " Do nothing for blacklisted files.
     if index(get(g:, 'ale_filetype_blacklist', []), l:filetype) >= 0
         return 1

test/test_should_do_nothing_conditions.vader

@@ -35,6 +35,10 @@ After:
   unlet! b:funky_command_created
   unlet! b:fake_mode
 
+  if &diff is 1
+    let &diff = 0
+  endif
+
   runtime autoload/ale/util.vim
 
 Given foobar(An empty file):
@@ -70,6 +74,11 @@ Execute(DoNothing should return 1 when an operator is pending):
 
   AssertEqual 1, ale#ShouldDoNothing(bufnr(''))
 
+Execute(DoNothing should return 1 for diff buffers):
+  let &diff = 1
+
+  AssertEqual 1, ale#ShouldDoNothing(bufnr(''))
+
 Execute(The DoNothing check should work if the ALE globals aren't defined):
   unlet! g:ale_filetype_blacklist
   unlet! g:ale_maximum_file_size