bin/get-pick-list.sh: sha1 commits can be smaller than 8 chars

The script only handles commits with "Fixes: <sha1>" where <sha1> is
equal or great than 8 chars. But <sha1> can be smaller, like 7 chars.

This commit relax the restriction to handle <sha1> 4 or more chars.

Fixes: 533fead4 ("bin/get-pick-list.sh: tweak the commit sha matching pattern")

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
(cherry picked from commit b3c25e6f)