Take the version from the git tag, not a constant (closes #80)

`mfer version`, `mfer --version` and the banner print the revision the
build stamps into main.Gitrev or, when it stamped none (as after
`go install` of a tag), the main module's version from
runtime/debug.ReadBuildInfo. The banner no longer shows a release date.

mfer.Version and mfer.ReleaseDate go, with main.Version and the version
field and parameter in internal/cli that nothing read. A program using
the library reads the module version from its own build information.
The README says where the version comes from.

Judgement call: a binary with no build information, or none holding a
module version, prints "unknown", the word script/build already uses
when git describe gives nothing.

Model: opus-5-5

cmd/mfer/main.go

@@ -10,14 +10,12 @@ import (
 // Appname is the name of this program.
 const Appname = "mfer"
 
-// Version and Gitrev are injected at build time via -ldflags.
+// Gitrev is the revision `mfer version` prints, injected at build time via
+// -ldflags.
 //
 //nolint:gochecknoglobals // set via ldflags at build time
-var (
-	Version string
-	Gitrev  string
-)
+var Gitrev string
 
 func main() {
-	os.Exit(cli.Run(Appname, Version, Gitrev))
+	os.Exit(cli.Run(Appname, Gitrev))
 }

cmd/mfer/main_test.go

@@ -15,11 +15,10 @@ func TestAppname(t *testing.T) {
 	assert.Equal(t, "mfer", Appname)
 }
 
-// TestVersionDefaults documents that Version and Gitrev are empty unless
-// injected at build time via -ldflags.
-func TestVersionDefaults(t *testing.T) {
+// TestGitrevDefault documents that Gitrev is empty unless injected at build
+// time via -ldflags.
+func TestGitrevDefault(t *testing.T) {
 	t.Parallel()
 
-	assert.Empty(t, Version)
 	assert.Empty(t, Gitrev)
 }