editor: file created for diff action should have .diff suffix

This is a follow-up to https://phab.mercurial-scm.org/D464 (6e6452bc441d) that
introduced the new file extension behavior. It erroneously changed `.diff` to
`.diff.hg.txt`.

Test Plan:
Verified `make tests` passes, particularly `test-editor-filename.t`.

Differential Revision: https://phab.mercurial-scm.org/D607

mercurial/ui.py

@@ -1361,7 +1361,9 b' class ui(object):'
             extra_defaults.update(extra)
         extra = extra_defaults
 
-        if action:
+        if action == 'diff':
+            suffix = '.diff'
+        elif action:
             suffix = '.%s.hg.txt' % action
         else:
             suffix = extra['suffix']

tests/test-editor-filename.t

@@ -33,3 +33,31 b' Verify that the path for a histedit edit'
   *.histedit.hg.txt (glob)
   abort: edit failed: editor.sh exited with status 1
   [255]
+
+Verify that when performing an action that has the side-effect of creating an
+editor for a diff, the file ends in .diff.
+
+  $ echo 1 > one
+  $ echo 2 > two
+  $ hg add
+  adding one
+  adding two
+  $ hg commit --interactive --config ui.interactive=true --config ui.interface=text << EOF
+  > y
+  > e
+  > q
+  > EOF
+  diff --git a/one b/one
+  new file mode 100644
+  examine changes to 'one'? [Ynesfdaq?] y
+  
+  @@ -0,0 +1,1 @@
+  +1
+  record change 1/2 to 'one'? [Ynesfdaq?] e
+  
+  *.diff (glob)
+  editor exited with exit code 1
+  record change 1/2 to 'one'? [Ynesfdaq?] q
+  
+  abort: user quit
+  [255]