Don't produce new subtrac commits under so many conditions.

This cleans up the resulting tree significantly, without any loss of
generality.

We also change the commit description to identify the source commit
(which by definition is one that's interesting because it changes a
submodule).

Author: apenwarr

subtrac.go

@@ -200,31 +200,38 @@ func (c *Cache) tracCommit(path string, commit *object.Commit) (*Trac, error) {
 
 	seenHeads := make(map[plumbing.Hash]bool)
 	seenTracs := make(map[plumbing.Hash]bool)
-	var heads []*Trac
+	var newHeads []*Trac
 	var tracs []*object.Commit
 
-	for _, h := range trac.subHeads {
-		if !seenHeads[h.hash] {
-			seenHeads[h.hash] = true
-			heads = append(heads, h)
-		}
-	}
 	for _, p := range trac.parents {
 		if p.tracCommit != nil {
 			if !seenTracs[p.tracCommit.Hash] {
 				seenTracs[p.tracCommit.Hash] = true
 				tracs = append(tracs, p.tracCommit)
 			}
 		}
+		for _, h := range p.subHeads {
+			// parent tracCommit already includes this subHead,
+			// so we don't need to include it again.
+			seenHeads[h.hash] = true
+		}
+	}
+	for _, h := range trac.subHeads {
+		if !seenHeads[h.hash] {
+			seenHeads[h.hash] = true
+			newHeads = append(newHeads, h)
+		}
 	}
 
-	if len(trac.parents) == 1 && equalSubs(trac.subHeads, trac.parents[0].subHeads) {
-		// Nothing has changed since our parent, no new commit needed.
-		trac.tracCommit = trac.parents[0].tracCommit
+	if len(newHeads) == 0 && len(tracs) <= 1 {
+		if len(tracs) == 1 {
+			// Nothing added since our parent, no new commit needed.
+			trac.tracCommit = tracs[0]
+		}
 	} else {
-		// Generate a new commit that includes our parent(s) and all
-		// our submodules.
-		trac.tracCommit, err = c.newTracCommit(commit, tracs, heads)
+		// Generate a new commit that includes our parent(s) and our
+		// new submodules.
+		trac.tracCommit, err = c.newTracCommit(commit, tracs, newHeads)
 		if err != nil {
 			return nil, err
 		}
@@ -279,12 +286,13 @@ func (c *Cache) newTracCommit(commit *object.Commit, tracs []*object.Commit, hea
 		return nil, fmt.Errorf("emptyTree.Store: %v", err)
 	}
 
+	msg := fmt.Sprintf("[git-subtrac for %v]", commit.Hash)
 	tc := &object.Commit{
 		Author:       sig,
 		Committer:    sig,
 		TreeHash:     emptyTreeHash,
 		ParentHashes: parents,
-		Message:      "[git-subtrac merge]",
+		Message:      msg,
 	}
 	nec = c.repo.Storer.NewEncodedObject()
 	err = tc.Encode(nec)