git: Allow local repos to use HEAD

git: Allow local repos to use HEAD

Introduce a new 'usehead' url parameter for git repositories. Specifying
usehead=1 causes bitbake to use whatever commit the repository HEAD is
pointing to. Usage of usehead=1 is only allowed for local git
repositories, i.e. it must always be accompanied with protocol=file url
parameter.

[YOCTO #9351]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>