bin/get-pick-list.sh: rework handing of sha nominations

Currently our is_sha_nomination does:
 - folds any whitespace, attempting to extract sha-like information
 - checks that at least one of the shas has landed

Split it in two and do sha-like validation first.

This way, commits with mesa-stable and sha nominations will feature the
fixes/revert/etc instead of stable (a) or will be omitted if not
applicable for the respective branch (b).

Misc examples from 18.3

(a)
-[   stable ] 5bc50936 glx: make xf86vidmode mandatory for direct rendering
+[    fixes ] 5bc50936 glx: make xf86vidmode mandatory for direct rendering

(b)
-[   stable ] 9a7b3199 anv/query: flush render target before copying results

CC: Juan A. Suarez <jasuarez@igalia.com>
CC: Dylan Baker <dylan@pnwbakers.com>
CC: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
(cherry picked from commit 6b296f64)