List time as builtin, support time --help

`a=b time foo` will no longer call an external `time` command
(like it does in bash).

Fixes #6598

src/builtin.cpp

@@ -394,6 +394,7 @@ static const builtin_data_t builtin_datas[] = {
     {L"string", &builtin_string, N_(L"Manipulate strings")},
     {L"switch", &builtin_generic, N_(L"Conditionally execute a block of commands")},
     {L"test", &builtin_test, N_(L"Test a condition")},
+    {L"time", &builtin_generic, N_(L"Measure how long a command or block takes")},
     {L"true", &builtin_true, N_(L"Return a successful result")},
     {L"ulimit", &builtin_ulimit, N_(L"Set or get the shells resource usage limits")},
     {L"wait", &builtin_wait, N_(L"Wait for background processes completed")},

src/parse_productions.cpp

@@ -397,16 +397,12 @@ RESOLVE(optional_background) {
 }
 
 RESOLVE(optional_time) {
-    UNUSED(token2);
-
-    switch (token1.keyword) {
-        case parse_keyword_time:
-            *out_tag = parse_optional_time_time;
-            return production_for<time>();
-        default:
-            *out_tag = parse_optional_time_no_time;
-            return production_for<empty>();
+    if (token1.keyword == parse_keyword_time && !token2.is_help_argument) {
+        *out_tag = parse_optional_time_time;
+        return production_for<time>();
     }
+    *out_tag = parse_optional_time_no_time;
+    return production_for<empty>();
 }
 
 const production_element_t *parse_productions::production_for_token(parse_token_type_t node_type,

tests/checks/time.fish

@@ -32,12 +32,6 @@ true && time a=b not builtin true | true
 #CHECKERR: {{.*}}
 #CHECKERR: {{.*}}
 
-PATH= time true
-#CHECKERR: fish: Unknown command: time
-#CHECKERR: {{.*}}
-#CHECKERR: PATH= time true
-#CHECKERR:       ^
-
 not time true
 #CHECKERR: ___{{.*}}
 #CHECKERR: {{.*}}