Escape trailing semicolon in tmux command argument

tmux ends a command at an argument whose last character is ';', so a
--border-label ending in one was stored truncated. escapeTmuxSeparator
only covered a value that was exactly ';'.

  fzf --popup --border-label 'foo;'   ->  @fzf-border-label was 'foo'

src/tmux.go

@@ -46,11 +46,11 @@ func tmuxFloatingPaneInfo() (int, int, bool) {
 	return width, height, true
 }
 
-// A lone ';' argument is a command separator to tmux, aborting the whole
-// command at parse time
+// tmux ends a command at an argument ending in ';', so a trailing one is
+// escaped. Elsewhere it is not special
 func escapeTmuxSeparator(str string) string {
-	if str == ";" {
-		return `\;`
+	if strings.HasSuffix(str, ";") {
+		return str[:len(str)-1] + `\;`
 	}
 	return str
 }

src/tmux_test.go

@@ -18,6 +18,7 @@ func TestEscapeTmuxFormat(t *testing.T) {
 		{" C# notes #S ", " C## notes ##S "},
 		{";", `\;`},
 		{"; rm", "; rm"},
+		{"C#;", `C##\;`},
 	} {
 		if actual := escapeTmuxFormat(tc.given); actual != tc.expected {
 			t.Errorf("expected %q, got %q", tc.expected, actual)
@@ -35,10 +36,15 @@ func TestEscapeTmuxSeparator(t *testing.T) {
 		{"#", "#"},
 		{"#{pane_id}", "#{pane_id}"},
 		{"100%", "100%"},
+		// tmux drops a trailing ';' and everything after it, so only that
+		// one is escaped
 		{";", `\;`},
+		{"abc;", `abc\;`},
+		{";;", `;\;`},
+		{`foo\;`, `foo\\;`},
 		{"; rm", "; rm"},
 		{" ; ", " ; "},
-		{";;", ";;"},
+		{"a;b", "a;b"},
 	} {
 		if actual := escapeTmuxSeparator(tc.given); actual != tc.expected {
 			t.Errorf("expected %q, got %q", tc.expected, actual)