git-undo: add page (#5980)

pages/common/git-undo.md

@@ -0,0 +1,13 @@
+# git undo
+
+> Undo recent commits.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-undo>.
+
+- Remove the most recent commit:
+
+`git undo`
+
+- Remove a specific number of the most recent commits:
+
+`git undo {{3}}`