scripts/functions: add git wrapper

Add a git wrapper to retrieve components from their git tree.
Add a git wrapper to create a working copy (in our tarballs dir).
Recognise git trees when searching for local copies.

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/dokuwiki/projects/crosstool"

reporter_comment=""