scripts/functions: Fix CT_ExtractGit function.

Change CT_ExtractGit so that it clones the repository, instead of just
symlinking it. After cloning, any given ref is checked out, or if no
ref is given, the HEAD of the repository is checked out.

This makes CT_Extract behave similar for git repositories as it does
for tarballs, so that it for example can be used for passing glibc-ports
as a git repository.

Signed-off-by: "Esben Haabendal" <esben.haabendal@prevas.dk>
[yann.morin.1998@anciens.enib.fr: fix incomplete var rename]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # This file adds functions to build the C library for bare metal (IE. none)

 # Copyright 2008 Yann E. MORIN

 # Licensed under the GPL v2. See COPYING in the root of this package

 do_libc_get() {

     :

 }

 do_libc_extract() {

     :

 }

 do_libc_check_config() {

     :

 }

 do_libc_start_files() {

     :

 }

 do_libc() {

     :

 }

 do_libc_finish() {

     :

 }