bin/get-pick-list.sh: prefix output with "[stable] "

bin/get-pick-list.sh: prefix output with "[stable] "

With later commits we'll fold all the different scripts into one.
Add the explicit prefix, so that we know the origin of the nomination

v2:
 - pass the sha as argument to the function
 - swap $tag = none for an else statment (Juan)
 - grep -q instead of using a variable (Eric)
 - print the tag and commit oneline separately (Eric)

v3:
 - drop unused "tag=none" assignment (Juan)
 - typo nomination

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> (v2)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>