bin/get-fixes-pick-list.sh: better identify multiple "fixes:" tags

bin/get-fixes-pick-list.sh: better identify multiple "fixes:" tags

We were not considering as multiple fixes lines with:
Fixes: $sha_1, Fixes: $sha_2
Now, we split the lines so we will consider them individually, as in:
Fixes: $sha_1,
Fixes: $sha_2
Additionally, we try to get the SHA from split lines so:
Fixes:
$sha_1

Will be considered as:
Fixes: $sha_1
v2:
 - Treat empty spaces earlier in fix lines (Emil)
 - Fold 2 lines into one to gather fix commit ids (Emil)

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Emil Velikov <emli.velikov@collabora.com>