git-pr: add page (#2674)

pages/common/git-pr.md

@@ -0,0 +1,19 @@
+# git pr
+
+> Check out GitHub pull requests locally.
+
+- Check out a specific pull request:
+
+`git pr {{pr_number}}`
+
+- Check out a pull request for a specific remote:
+
+`git pr {{pr_number}} {{remote}}`
+
+- Check out a pull request from its URL:
+
+`git pr {{url}}`
+
+- Clean up old pull request branches:
+
+`git pr clean`