internal: small optimization on UniqueTags.
internal: small optimization on UniqueTags.

Change-Id: Id46e54b98ab40e33037a3d99623c7c194ecf4249
Reviewed-on: https://go-review.googlesource.com/17221
Reviewed-by: Ralph Corderoy <d6424ecc823ca663bdcbaea8e2d9df58a4d0e2b8@inputplus.co.uk>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

cmars/tools,cmars/tools,golang/text,cmars/tools,cmars/tools