git-root: add page (#6009)

pages/common/git-root.md

@@ -0,0 +1,13 @@
+# git root
+
+> Print the root directory of the current Git repository.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-root>.
+
+- Print the absolute path of the current Git repository:
+
+`git root`
+
+- Print the current working directory relative to the root of the current Git repository:
+
+`git root --relative`