Add change-header-lines action to dynamically change --header-lines

All input lines now enter the chunklist with sequential indices, and
header lines are excluded from matching via Pattern.startIndex and
PassMerger offset. This allows the number of header lines to be changed
at runtime with change-header-lines(N), transform-header-lines, and
bg-transform-header-lines actions.

- Remove EvtHeader event; header items are read directly from chunks
- Add startIndex to Pattern and PassMerger for skipping header items
- Add targetIndex field to Terminal for cursor repositioning across
  header-lines changes

Close #4659

src/merger.go

@@ -10,42 +10,46 @@ func EmptyMerger(revision revision) *Merger {
 // Merger holds a set of locally sorted lists of items and provides the view of
 // a single, globally-sorted list
 type Merger struct {
-	pattern  *Pattern
-	lists    [][]Result
-	merged   []Result
-	chunks   *[]*Chunk
-	cursors  []int
-	sorted   bool
-	tac      bool
-	final    bool
-	count    int
-	pass     bool
-	revision revision
-	minIndex int32
-	maxIndex int32
+	pattern    *Pattern
+	lists      [][]Result
+	merged     []Result
+	chunks     *[]*Chunk
+	cursors    []int
+	sorted     bool
+	tac        bool
+	final      bool
+	count      int
+	pass       bool
+	startIndex int
+	revision   revision
+	minIndex   int32
+	maxIndex   int32
 }
 
 // PassMerger returns a new Merger that simply returns the items in the
-// original order
-func PassMerger(chunks *[]*Chunk, tac bool, revision revision) *Merger {
+// original order. startIndex items are skipped from the beginning.
+func PassMerger(chunks *[]*Chunk, tac bool, revision revision, startIndex int32) *Merger {
 	var minIndex, maxIndex int32
 	if len(*chunks) > 0 {
 		minIndex = (*chunks)[0].items[0].Index()
 		maxIndex = (*chunks)[len(*chunks)-1].lastIndex(minIndex)
 	}
+	si := int(startIndex)
 	mg := Merger{
-		pattern:  nil,
-		chunks:   chunks,
-		tac:      tac,
-		count:    0,
-		pass:     true,
-		revision: revision,
-		minIndex: minIndex,
-		maxIndex: maxIndex}
+		pattern:    nil,
+		chunks:     chunks,
+		tac:        tac,
+		count:      0,
+		pass:       true,
+		startIndex: si,
+		revision:   revision,
+		minIndex:   minIndex + startIndex,
+		maxIndex:   maxIndex}
 
 	for _, chunk := range *mg.chunks {
 		mg.count += chunk.count
 	}
+	mg.count = max(0, mg.count-si)
 	return &mg
 }
 
@@ -113,6 +117,7 @@ func (mg *Merger) Get(idx int) Result {
 		if mg.tac {
 			idx = mg.count - idx - 1
 		}
+		idx += mg.startIndex
 		firstChunk := (*mg.chunks)[0]
 		if firstChunk.count < chunkSize && idx >= firstChunk.count {
 			idx -= firstChunk.count