add option to lint diff buffers

autoload/ale.vim

@@ -48,7 +48,7 @@ function! ale#ShouldDoNothing(buffer) abort
     endif
 
     " Do nothing for diff buffers.
-    if getbufvar(a:buffer, '&diff')
+    if getbufvar(a:buffer, '&diff') && !get(g:, 'ale_lint_diff', 0)
         return 1
     endif
 

doc/ale.txt

@@ -1620,10 +1620,21 @@ g:ale_lint_delay
   A buffer-local option, `b:ale_lint_delay`, can be set to change the delay
   for different buffers, such as in |ftplugin| files.
 
+                                                    *ale-options.ale_lint_diff*
+                                                              *g:ale_lint_diff*
+g:ale_lint_diff
+
+  Type: |Number|
+  Default: `0`
+
+  When this option is set to `1`, ALE will lint buffers where `&diff` is set.
+
+
                                                     *ale-options.lint_on_enter*
                                                           *g:ale_lint_on_enter*
 lint_on_enter
 g:ale_lint_on_enter
+
   Type: |Number|
   Default: `1`
 

test/test_should_do_nothing_conditions.vader

@@ -32,6 +32,7 @@ After:
     let b:funky_command_created = 0
   endif
 
+  unlet! g:ale_lint_diff
   unlet! b:funky_command_created
   unlet! b:fake_mode
 
@@ -79,6 +80,12 @@ Execute(DoNothing should return 1 for diff buffers):
 
   AssertEqual 1, ale#ShouldDoNothing(bufnr(''))
 
+Execute(DoNothing should return 0 for diff buffers when ale_lint_diff is set):
+  let &diff = 1
+  let g:ale_lint_diff = 1
+
+  AssertEqual 0, 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