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>

# x86 specific options

## select ARCH_SUPPORTS_32

## select ARCH_SUPPORTS_64

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORT_ARCH

## select ARCH_SUPPORT_CPU

## select ARCH_SUPPORT_TUNE

##

## help The x86 architecture, as defined by:

## help     32-bit (ia32) : http://www.intel.com/

## help     64-bit (amd64): http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html