git-update-index: add page (#3911)

pages/common/git-update-index.md

@@ -0,0 +1,8 @@
+# git update-index
+
+> Git command for manipulating the index.
+> More information: <https://git-scm.com/docs/git-update-index>.
+
+- Pretend that a modified file is unchanged (`git status` will not show this as changed):
+
+`git update-index --skip-worktree {{path/to/modified_file}}`