A commit is never a branch to follow (hq issue 215)

A build asked at a commit recorded that commit as the module's ref. Every merge after it failed to
match the module and its plan left it out without a word, and every plan that rebuilt it asked for
the same old commit again. Registration now keeps the branch the module followed (the default
branch for a new one); matching and re-asking read a recorded commit as the default branch, which
heals records already pinned this way; and a merge says which modules of its repository it leaves
out because they follow another branch.

cmd/mesh-controller/upgrades.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"regexp"
 	"strings"
 	"time"
 
@@ -283,6 +284,14 @@ func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
 	if isHistory(m.MergedAt, lastLookAt(entries, m)) {
 		packaging = nil
 	}
+	// Said, never silent (novox/hq 04-ISSUES/215): a module built from this repository that follows
+	// another branch is not part of this merge, and whoever is waiting for its change should read why.
+	for _, e := range entries {
+		if sameRepository(e.Source.Repository, m) && !sourceIs(e.Source, m) {
+			fmt.Printf("  %s is built from %s/%s and follows %s, not %s; this merge leaves it out\n",
+				e.Manifest.Module, m.Owner, m.Repo, e.Source.Ref, m.Base)
+		}
+	}
 	touched := whatTheMergeTouched(from, entries, m)
 	for _, e := range touched {
 		if err := inv.SourceMoved(ctx, e.Manifest.Module, m.Commit); err != nil {
@@ -345,7 +354,24 @@ func sourceIs(s inventory.Source, m link.SourceMoved) bool {
 	if !sameRepository(s.Repository, m) {
 		return false
 	}
-	return s.Ref == "" || s.Ref == m.Base
+	ref := followedBranch(s.Ref)
+	return ref == "" || ref == m.Base
+}
+
+// commitRef is a ref that names a commit rather than a branch: what `build --ref <commit>` asks for.
+var commitRef = regexp.MustCompile(`^[0-9a-f]{7,40}$`)
+
+// followedBranch is the branch a recorded ref means a module follows (novox/hq 04-ISSUES/215). **A
+// commit is never a branch to follow.** A build asked at a commit — to try one, or to pin it during a
+// fix — recorded that commit as the module's ref; every merge after it then failed to match the
+// module, its plan left it out without saying so, and every plan that rebuilt it asked for that same
+// old commit again. A commit recorded so is read as the repository's default branch, which is what
+// the module followed before it; a branch is followed as named.
+func followedBranch(ref string) string {
+	if commitRef.MatchString(strings.TrimSpace(ref)) {
+		return ""
+	}
+	return ref
 }
 
 // sameRepository is whether a recorded repository is the one a merge names, in either spelling it