Show git commit and build time in vintner version

Uses Go's automatic VCS build-info stamping (vcs.revision/vcs.time/
vcs.modified, embedded by `go build` since Go 1.18 - no -ldflags
changes needed, works the same for a CI release build and a plain
local `go build`). Knowing the exact commit a bug report's binary was
built from, not just the X.Y.Z tag, is the point - two builds of the
same tag could still differ.

Split the pure formatting logic (formatVersion) from the
debug.ReadBuildInfo() call so it's actually unit-testable: `go test`
binaries don't get VCS stamping the way `go build` ones do, so there
was no way to exercise the revision-formatting branch through
versionString() itself.

cmd/vintner/version.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+	"runtime/debug"
+)
+
+// versionString renders "vintner <version>" plus, when available, the git
+// commit and build time Go's toolchain embeds automatically (since Go
+// 1.18, `go build` stamps vcs.revision/vcs.time/vcs.modified into the
+// binary on its own - no -ldflags needed for this part, so it works the
+// same whether the binary came from CI or a plain local `go build`).
+// Knowing the exact commit a reported bug was built from, not just the
+// X.Y.Z tag, is the point: two builds of the same tag could still differ
+// if the tag was ever moved, or if someone built from an uncommitted tree.
+//
+// Note for anyone testing this: `go build` stamps vcs.* build settings,
+// but `go test` binaries don't get them - there's no environment where a
+// `go test` run can exercise the revision-formatting branch below, hence
+// formatVersion is split out and tested directly instead.
+func versionString() string {
+	revision, buildTime, dirty := "", "", false
+	if info, ok := debug.ReadBuildInfo(); ok {
+		for _, s := range info.Settings {
+			switch s.Key {
+			case "vcs.revision":
+				revision = s.Value
+			case "vcs.time":
+				buildTime = s.Value
+			case "vcs.modified":
+				dirty = s.Value == "true"
+			}
+		}
+	}
+	return formatVersion(version, revision, buildTime, dirty)
+}
+
+// formatVersion is the pure part of versionString: given a revision (full
+// git SHA, may be empty), it's shortened to 12 chars and marked "-dirty" if
+// the build tree had uncommitted changes.
+func formatVersion(ver, revision, buildTime string, dirty bool) string {
+	v := "vintner " + ver
+	if revision == "" {
+		return v
+	}
+	if len(revision) > 12 {
+		revision = revision[:12]
+	}
+	if dirty {
+		revision += "-dirty"
+	}
+	if buildTime != "" {
+		return fmt.Sprintf("%s (%s, %s)", v, revision, buildTime)
+	}
+	return fmt.Sprintf("%s (%s)", v, revision)
+}