Implement bindable "change" event and "top" action

# Move cursor to the top result whenever the query string is changed
    fzf --bind change:top

Close #925

man/man1/fzf.1

@@ -467,6 +467,11 @@ e.g. \fBfzf --bind=ctrl-j:accept,ctrl-k:kill-line\fR
     \fIdouble-click\fR
     or any single character
 
+Additionally, a special event named \fIchange\fR is available which is
+triggered whenever the query string is changed.
+
+    e.g. \fBfzf --bind change:top\fR
+
   \fBACTION:               DEFAULT BINDINGS (NOTES):
     \fBabort\fR                 \fIctrl-c  ctrl-g  ctrl-q  esc\fR
     \fBaccept\fR                \fIenter   double-click\fR
@@ -513,6 +518,7 @@ e.g. \fBfzf --bind=ctrl-j:accept,ctrl-k:kill-line\fR
     \fBtoggle-preview-wrap\fR
     \fBtoggle-sort\fR
     \fBtoggle+up\fR             \fIbtab    (shift-tab)\fR
+    \fBtop\fR                   (move to the top result)
     \fBunix-line-discard\fR     \fIctrl-u\fR
     \fBunix-word-rubout\fR      \fIctrl-w\fR
     \fBup\fR                    \fIctrl-k  ctrl-p  up\fR

src/options.go

@@ -400,6 +400,8 @@ func parseKeyChords(str string, message string) map[int]string {
 			chord = tui.BSpace
 		case "ctrl-space":
 			chord = tui.CtrlSpace
+		case "change":
+			chord = tui.Change
 		case "alt-enter", "alt-return":
 			chord = tui.CtrlAltM
 		case "alt-space":
@@ -714,6 +716,8 @@ func parseKeymap(keymap map[int][]action, str string) {
 				appendAction(actDown)
 			case "up":
 				appendAction(actUp)
+			case "top":
+				appendAction(actTop)
 			case "page-up":
 				appendAction(actPageUp)
 			case "page-down":

src/terminal.go

@@ -213,6 +213,7 @@ const (
 	actExecuteSilent
 	actExecuteMulti // Deprecated
 	actSigStop
+	actTop
 )
 
 func toActions(types ...actionType) []action {
@@ -1609,6 +1610,9 @@ func (t *Terminal) Loop() {
 				req(reqClose)
 			case actClearScreen:
 				req(reqRedraw)
+			case actTop:
+				t.vset(0)
+				req(reqList)
 			case actUnixLineDiscard:
 				if t.cx > 0 {
 					t.yanked = copySlice(t.input[:t.cx])
@@ -1749,6 +1753,11 @@ func (t *Terminal) Loop() {
 			}
 			t.truncateQuery()
 			changed = string(previousInput) != string(t.input)
+			if onChanges, prs := t.keymap[tui.Change]; changed && prs {
+				if !doActions(onChanges, tui.Change) {
+					continue
+				}
+			}
 		} else {
 			if mapkey == tui.Rune {
 				if idx := strings.IndexRune(t.jumpLabels, event.Char); idx >= 0 && idx < t.maxItems() && idx < t.merger.Length() {

src/tui/tui.go

@@ -75,6 +75,8 @@ const (
 	F11
 	F12
 
+	Change
+
 	AltSpace
 	AltSlash
 	AltBS

test/test_go.rb

@@ -1304,6 +1304,20 @@ class TestGoFZF < TestBase
     tmux.send_keys :Enter
     tmux.until { |lines| lines[-2] == prompt && lines[-1] == '1' }
   end
+
+  def test_change_top
+    tmux.send_keys %[seq 1000 | #{FZF} --bind change:top], :Enter
+    tmux.until { |lines| lines.match_count == 1000 }
+    tmux.send_keys :Up
+    tmux.until { |lines| lines[-4] == '> 2' }
+    tmux.send_keys 1
+    tmux.until { |lines| lines[-3] == '> 1' }
+    tmux.send_keys :Up
+    tmux.until { |lines| lines[-4] == '> 10' }
+    tmux.send_keys 1
+    tmux.until { |lines| lines[-3] == '> 11' }
+    tmux.send_keys :Enter
+  end
 end
 
 module TestShell