[MERGE] Do not scan history for tags when none are present.

[Robert Collins]

This improves the time for 'bzr tags' when no tags are present:

robertc at lifeless-64:~/source/baz/misc-fixen$ time ./bzr tags

real    0m0.401s
user    0m0.292s
sys     0m0.064s


robertc at lifeless-64:~/source/baz/misc-fixen$ time ../bzr.dev/bzr tags

real    0m8.449s
user    0m3.144s
sys     0m0.160s


It doesn't help when tags are needed; the problem is that bzr is
generating a full revision map.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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