Fatal: Expand $TAGS{':all'} immediately

_expand_tags is not really suited for expanding ":all" due to how it
was constructed.  However, it turns out that it is quite trivial to
expand ":all"-case is a reasonably more efficient manner.

Since ":all" is now always expanded, we can abuse this to reuse the
value in %tag_cache (_expand_tag).

Signed-off-by: Niels Thykier <niels@thykier.net>