support/download: relocate the git clone comment

Following commit 95a57228 (pkg-infra: move the git download helper to a
script, 2014-07-02), move the comment describing the shallow clone trickery as
well. Merge this comment with the existing helper comment that was added in
7e40a110 (support/download: convert git to use the wrapper, 2014-08-03).

Rename $($(PKG)_DL_VERSION) to ${cset} to match the helper code context.

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>