handle git

env GIT_AUTHOR_NAME='Gopher'
env GIT_AUTHOR_EMAIL='gopher@golang.org'
env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL

git init
git checkout -b master

at 2018-01-01T10:00:00-05:00
git add prefercompatible.go
git commit -m 'v1.5.1'
git tag v1.5.1

at 2018-01-02T10:00:00-05:00
cp go.mod.v1 go.mod
git add go.mod
git commit -m 'add go.mod'
git tag v1.5.2

at 2018-01-03T10:00:00-05:00
git checkout v1.5.1
git commit --allow-empty -m 'v2'
git tag v2.0.0

at 2018-01-04T10:00:00-05:00
cp go.mod.v2 go.mod
git add go.mod
git commit -m 'add v2 go.mod'
git tag v2.0.1

at 2018-01-05T10:00:00-05:00
git commit --allow-empty -m 'v2.1'
git tag v2.1.0

git checkout master

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

-- .git-refs --
17354ea3a81eb684eb1aae80a7b52a5034bd34d2 refs/heads/master
e79838949df193e551567d3f245720f7a6520569 refs/tags/v1.5.1
17354ea3a81eb684eb1aae80a7b52a5034bd34d2 refs/tags/v1.5.2
7a6f62732689a8e013c3ab4e5e0566ab4969a46e refs/tags/v2.0.0
45f40e4783d7a9908c85e6dc48603fce3dd6ffad refs/tags/v2.0.1
70d729681ff71f5dd03dd7142c78fb98f35065fe refs/tags/v2.1.0
-- prefercompatible.go --
package prefercompatible
-- go.mod.v1 --
module vcs-test.golang.org/git/prefercompatible.git

go 1.12
-- go.mod.v2 --
module vcs-test.golang.org/git/prefercompatible.git/v2

go 1.12