externalsrc: Always ask Git for location of .git directory

externalsrc: Always ask Git for location of .git directory

externalsrc_configure_prefunc assumed that the .git directory is
${S}/.git. This isn't true for submodules at least.

srctree_hash_files already contained code to ask Git for the correct
path to the .git directory. Let's move that code to a new find_git_dir
function and call it from both places and make the behaviour consistent.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>