[PATCH 0 of 3 STABLE] tag: bug fix around overwriting each other between global and local tags

[FUJIWARA Katsunori]

This patch series focuses on consistent view of tags in the case of
overwriting each other between global and local tags.

For someone expecting that local tag can overwrite existing global
one, this series breaks backward compatibility. But IMHO, for
consistent view of tags, local tag should be prevented from
overwriting global one.

Perhaps, patch #3 may not be suitable for stable branch, because this
patch looks like a kind of improvement rather than bug fix.