git checkout master
git merge b -m 'merge'

git show-ref --tags --heads
cmp stdout .git-refs

-- .git-refs --
59356c8cd18c5fe9a598167d98a6843e52d57952 refs/heads/b
c7818c24fa2f3f714c67d0a6d3e411c85a518d1f refs/heads/master
101c49f5af1b2466332158058cf5f03c8cca6429 refs/tags/v0.2.1
59356c8cd18c5fe9a598167d98a6843e52d57952 refs/tags/v0.2.2
-- go.mod --
module vcs-test.golang.org/git/tagtests.git
-- tagtests.go --
package tagtests

stdout '"VCS": "git"'
stdout '"URL": ".*/git/tagtests"'
! stdout '"TagPrefix"'
stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
stdout '"Ref": "refs/tags/v0.2.2"'
stdout '"Hash": "59356c8cd18c5fe9a598167d98a6843e52d57952"'

# go mod download vcstest/tagtests@v0.2.2 should print origin info, no TagSum needed
go mod download -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
cp stdout tagtestsv022.json