Merge branch 'cw/tag-reflog-message'

"git tag" did not leave useful message when adding a new entry to
reflog; this was left unnoticed for a long time because refs/tags/*
doesn't keep reflog by default.

* cw/tag-reflog-message:
tag: generate useful reflog message

diff --cc builtin/tag.c
Simple merge

diff --cc t/t7004-tag.sh
Simple merge