git-mr: add page (#7585)

pages/common/git-mr.md

@@ -0,0 +1,21 @@
+# git mr
+
+> Check out GitLab merge requests locally.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-mr>.
+
+- Check out a specific merge request:
+
+`git mr {{mr_number}}`
+
+- Check out a merge request from a specific remote:
+
+`git mr {{mr_number}} {{remote}}`
+
+- Checkout a merge request from its URL:
+
+`git mr {{url}}`
+
+- Clean up old merge request branches:
+
+`git mr clean`