fetch/git: download LFS content too during do_fetch

Insert an explicit pass to fetch all blobs needed by Git LFS, during the
fetch() function. This avoids the default behavior of Git LFS to wait
until 'git checkout' to begin downloading the blobs pointed to by LFS records.
Network access is not allowed at that point in the recipe's lifecycle.

[YOCTO #14191]

Signed-off-by: Matt Hoosier <matt.hoosier@garmin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>