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

test/test_filter.rb

@@ -326,4 +326,27 @@ class TestFilter < TestBase
     writelines(['emp001 Alice Engineering', 'emp002 Bob Marketing'])
     assert_equal 'emp001', `#{FZF} -d' ' --with-nth 2 --accept-nth 1 -f Alice < #{tempname}`.chomp
   end
+
+  def test_header_lines_filter
+    assert_equal %w[4 5 6 7 8 9 10],
+                 `seq 10 | #{FZF} --header-lines 3 -f ""`.lines(chomp: true)
+    assert_equal %w[5],
+                 `seq 10 | #{FZF} --header-lines 3 -f 5`.lines(chomp: true)
+    # Header items should not be matched
+    assert_empty `seq 10 | #{FZF} --header-lines 3 -f "^1$"`.lines(chomp: true)
+  end
+
+  def test_header_lines_filter_with_nth
+    writelines(%w[a:1 b:2 c:3 d:4 e:5])
+    assert_equal %w[c:3 d:4 e:5],
+                 `#{FZF} --header-lines 2 -d: --with-nth 2 -f "" < #{tempname}`.lines(chomp: true)
+    assert_equal %w[d:4],
+                 `#{FZF} --header-lines 2 -d: --with-nth 2 -f 4 < #{tempname}`.lines(chomp: true)
+  end
+
+  def test_header_lines_all_headers
+    # When all lines are header lines, no results
+    assert_empty `seq 3 | #{FZF} --header-lines 10 -f ""`.chomp
+    assert_equal 1, $CHILD_STATUS.exitstatus
+  end
 end