pkg-download.mk: fix multiple matches in shallow clone test

Move the git reference directly to git-ls-remote rather than
piping the output to grep, to help avoid reporting partial
matches rather than the actual requested reference.  Also,
add quotes to protest "test" from failing when multiple
strings are reported.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>