Make --expect additive

Similarly to --bind or --color.

--expect used to replace the previously specified keys, and
fzf#wrap({'options': '--expect=f1'}) wouldn't work as expected. It
forced us to come up with some ugly hacks like the following:

https://github.com/junegunn/fzf.vim/blob/13b27c45c8bdf6c3a41376bb83e4895edadf8c7e/autoload/fzf/vim.vim#L1086

man/man1/fzf.1

@@ -331,10 +331,12 @@ Comma-separated list of keys that can be used to complete fzf in addition to
 the default enter key. When this option is set, fzf will print the name of the
 key pressed as the first line of its output (or as the second line if
 \fB--print-query\fR is also used). The line will be empty if fzf is completed
-with the default enter key.
+with the default enter key. If \fB--expect\fR option is specified multiple
+times, fzf will expect the union of the keys. \fB--no-expect\fR will clear the
+list.
 
 .RS
-e.g. \fBfzf --expect=ctrl-v,ctrl-t,alt-s,f1,f2,~,@\fR
+e.g. \fBfzf --expect=ctrl-v,ctrl-t,alt-s --expect=f1,f2,~,@\fR
 .RE
 .TP
 .B "--read0"

src/options.go

@@ -962,7 +962,9 @@ func parseOptions(opts *Options, allArgs []string) {
 		case "--algo":
 			opts.FuzzyAlgo = parseAlgo(nextString(allArgs, &i, "algorithm required (v1|v2)"))
 		case "--expect":
-			opts.Expect = parseKeyChords(nextString(allArgs, &i, "key names required"), "key names required")
+			for k, v := range parseKeyChords(nextString(allArgs, &i, "key names required"), "key names required") {
+				opts.Expect[k] = v
+			}
 		case "--no-expect":
 			opts.Expect = make(map[int]string)
 		case "--tiebreak":
@@ -1140,7 +1142,9 @@ func parseOptions(opts *Options, allArgs []string) {
 			} else if match, value := optString(arg, "--toggle-sort="); match {
 				parseToggleSort(opts.Keymap, value)
 			} else if match, value := optString(arg, "--expect="); match {
-				opts.Expect = parseKeyChords(value, "key names required")
+				for k, v := range parseKeyChords(value, "key names required") {
+					opts.Expect[k] = v
+				}
 			} else if match, value := optString(arg, "--tiebreak="); match {
 				opts.Criteria = parseTiebreak(value)
 			} else if match, value := optString(arg, "--color="); match {

src/options_test.go

@@ -414,3 +414,10 @@ func TestPreviewOpts(t *testing.T) {
 		t.Error(opts.Preview)
 	}
 }
+
+func TestAdditiveExpect(t *testing.T) {
+	opts := optsFor("--expect=a", "--expect", "b", "--expect=c")
+	if len(opts.Expect) != 3 {
+		t.Error(opts.Expect)
+	}
+}