support/download: fetch all refs on full git clone

When specifying BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION, a user may want to
specify the SHA of a reference different than a branch or tag.

For instance, Gerrit stores the patchsets under refs/changes/xx/xxx, and
Github stores the pull requests under refs/pull/xxx/head.

When cloning a repository with --bare, you don't fetch these references.
This patch uses --mirror for a full clone, in order to give the user
access to all references of the Git repository.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: "Maxime Hadjinlian" <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>