bin/get-fixes-pick-list.sh: bring back the warning

bin/get-fixes-pick-list.sh: bring back the warning

We warn again if there are more than one line with the "fixes:" tag.

The warning is silenced when the commit has already landed or each
fixes tag reference a commit that is in branch.

v2:
 - Warn if any of the fixes tags has not landed (Emil)

v3:
 - Remove unnecessary head command
 - Clarify commit message (Emil)
 - Skip already picked commits sooner (Emil)

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