diffstat: add page

----
- [x] The page (if new), does not already exist in the repo.

- [x] The page (if new), has been added to the correct platform folder:  
      `common/` if it's common to all platforms, `linux/` if it's Linux-specific, and so on.

- [x] The page has 8 or fewer examples.

- [ x The PR is appropriately titled:  
      `<command name>: add page` for new pages, or `<command name>: <description of changes>` for pages being edited

- [x] The page follows the [contributing](https://github.com/tldr-pages/tldr/blob/master/CONTRIBUTING.md) guidelines

pages/common/diffstat.md

@@ -0,0 +1,11 @@
+# diffstat
+
+> Make histogram of diff-output
+
+- Display changes in a histogram:
+
+`diff {{file1}} {{file2}} | diffstat`
+
+- Display inserted, deleted, modified changes in table format:
+
+`diff {{file1}} {{file2}} | diffstat -t`