Set --border-label as the title of the tmux floating pane (#4853)

The pane sets the options itself before running fzf, so that they are
in place no matter how quickly the command exits, targeted at
$TMUX_PANE; the default target would resolve to the active pane of
the session's current window.

- pane-border-format is set to '#{pane_title}' so that the label is
  displayed on the border when pane-border-status is enabled;
  pane-border-status itself is a window option in released tmux
  versions and is left alone
- When a border style is explicitly specified with --border, a popup
  is used instead of a floating pane so that the fzf-drawn border is
  the only border shown; give 'border-native' to force a floating pane
- 'none' and 'line' are treated as no border; fzf draws no box for
  either, so the label is displayed on the native border
- Remove 'border-fzf' which is now redundant; it was never released
- The title is escaped for select-pane -T which expands format
  expressions; a lone ';' is escaped as tmux would treat it as a
  command separator
- The label is skipped when ANSI stripping leaves an empty string
- --border-label-pos is ignored
- Fix remain-on-exit set on the original pane instead of the floating
  pane

src/tmux_test.go

@@ -0,0 +1,39 @@
+package fzf
+
+import "testing"
+
+func TestEscapeTmuxTitle(t *testing.T) {
+	for _, tc := range []struct {
+		given    string
+		expected string
+	}{
+		{"", ""},
+		{" fzf ", " fzf "},
+		{"#", "##"},
+		{"##", "####"},
+		{" C# notes #S ", " C## notes ##S "},
+		{"100%", "100%"},
+		{";", `\;`},
+		{"; rm", "; rm"},
+		{" ; ", " ; "},
+	} {
+		if actual := escapeTmuxTitle(tc.given); actual != tc.expected {
+			t.Errorf("expected %q, got %q", tc.expected, actual)
+		}
+	}
+}
+
+func TestEscapeTmuxTitleSeparator(t *testing.T) {
+	for _, tc := range []struct {
+		given    string
+		expected string
+	}{
+		{"#;", "##;"},
+		{";#", ";##"},
+		{";;", ";;"},
+	} {
+		if actual := escapeTmuxTitle(tc.given); actual != tc.expected {
+			t.Errorf("expected %q, got %q", tc.expected, actual)
+		}
+	}
+}