Simplify code and fix comment

The "// merge commit" comment was plain wrong, this is any commit that has a
parent, merge or not. The "else if" condition was unnecessary, a plain "else"
would have been enough. But the code in the two blocks was almost identical, so
extract the one thing that was different and unify it.

And while we're at it, use IsFirstCommit() instead of counting parents.

pkg/gui/presentation/graph/graph.go

@@ -133,25 +133,20 @@ func getNextPipes(prevPipes []*Pipe, commit *models.Commit, getStyle func(c *mod
 	// a traversed spot is one where a current pipe is starting on, ending on, or passing through
 	traversedSpots := set.New[int]()
 
-	if len(commit.Parents) > 0 { // merge commit
-		newPipes = append(newPipes, &Pipe{
-			fromPos:  pos,
-			toPos:    pos,
-			fromHash: commit.Hash,
-			toHash:   commit.Parents[0],
-			kind:     STARTS,
-			style:    getStyle(commit),
-		})
-	} else if len(commit.Parents) == 0 { // root commit
-		newPipes = append(newPipes, &Pipe{
-			fromPos:  pos,
-			toPos:    pos,
-			fromHash: commit.Hash,
-			toHash:   models.EmptyTreeCommitHash,
-			kind:     STARTS,
-			style:    getStyle(commit),
-		})
+	var toHash string
+	if commit.IsFirstCommit() {
+		toHash = models.EmptyTreeCommitHash
+	} else {
+		toHash = commit.Parents[0]
 	}
+	newPipes = append(newPipes, &Pipe{
+		fromPos:  pos,
+		toPos:    pos,
+		fromHash: commit.Hash,
+		toHash:   toHash,
+		kind:     STARTS,
+		style:    getStyle(commit),
+	})
 
 	traversedSpotsForContinuingPipes := set.New[int]()
 	for _, pipe := range currentPipes {