git-lock: add page (#6756)

pages/common/git-lock.md

@@ -0,0 +1,9 @@
+# git lock
+
+> Lock a file in a Git repository from being modified by a commit.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-lock>.
+
+- Disable the ability to commit changes of a local file:
+
+`git lock {{path/to/file}}`