If there is no parameter given by the user, use `<arg>`
instead.
+--prefix <arg>::
+ Behave as if 'git rev-parse' was invoked from the `<arg>`
+ subdirectory of the working tree. Any relative filenames are
+ resolved as if they are prefixed by `<arg>` and will be printed
+ in that form.
++
+This can be used to convert arguments to a command run in a subdirectory
+so that they can still be used after moving to the top-level of the
+repository. For example:
++
+----
+prefix=$(git rev-parse --show-prefix)
+cd "$(git rev-parse --show-toplevel)"
+eval "set -- $(git rev-parse --sq --prefix "$prefix" "$@")"
+----
+
--verify::
Verify that exactly one parameter is provided, and that it
can be turned into a raw 20-byte SHA-1 that can be used to