Fix "--" parsing in NestedCommandLineApp

Summary:
Adjusts the parsing order so that folly doesn't incorrectly consume multiple "--" arguments.

# -- in unix

Folly is a facebook library for many common C++ operations. It's open-source.

"--" is a unix convention ([example](https://unix.stackexchange.com/questions/11376/what-does-double-dash-mean)). It means: Anything after "--" should be parsed as a positional argument. This allows you to nest CLIs together.

For example "buck run" is a CLI, with various options. The binary that is built may have options. When you want to pass something to the underlying binary, rather than buck run, you can use "--":

```
buck run //my:binary --buck-option 1 -- --mybinary-option 2
```

This prevents buck from accidentally thinking "--mybinary-option" is an arg that should be parsed by the buck binary.

One such positional argument that can occur is "--" itself. In this case, the first "--" tells the binary to treat every subsequent arg as a positional arg, and the next "--" is interpreted **as a positional arg**:

```
buck run -- --wrapper-option -- --inner-option
```

The positional args here are:
["--wrapper-option", "--", "--inner-option"]

This allows for an arbitrary number of wrappers to be chained together.

# What's wrong / The Fix

The current logic will never allow a "--" to be passed as a positional argument, as it does not consider if it's already seen a "--". We adjust the order to:
1. If we've already seen "--" ALWAYS treat the arg as a positional arg
2. If it's the first "--", consume it. All subsequent args are positional.
3. If we haven't yet seen "--", it should be included when looking for options.

That way, we don't accidentally delete a "--" that's intended for an underlying parser.

Reviewed By: simpkins

Differential Revision: D44892191

fbshipit-source-id: 58ff1a993cd68e0a65aa039bee588bc659c8ad8d

folly/experimental/NestedCommandLineApp.cpp

@@ -284,10 +284,10 @@ void NestedCommandLineApp::doRun(const std::vector<std::string>& args) {
   std::vector<std::string> endArgs;
 
   for (auto& na : args) {
-    if (na == "--") {
-      not_clean = true;
-    } else if (not_clean) {
+    if (not_clean) {
       endArgs.push_back(na);
+    } else if (na == "--") {
+      not_clean = true;
     } else {
       cleanArgs.push_back(na);
     }

folly/experimental/test/NestedCommandLineAppTest.cpp

@@ -109,19 +109,28 @@ TEST(ProgramOptionsTest, DevFull) {
 }
 
 TEST(ProgramOptionsTest, CutArguments) {
-  // anything after -- is parsed as arguments
+  // anything after -- is parsed as arguments, including more --
   EXPECT_EQ(
       "running foo\n"
       "foo global-foo 43\n"
       "foo local-foo 42\n"
       "foo conflict-global 42\n"
       "foo conflict 42\n"
       "foo arg b\n"
+      "foo arg --\n"
       "foo arg --local-foo\n"
       "foo arg 44\n"
       "foo arg a\n",
       callHelper(
-          {"foo", "--global-foo", "43", "--", "b", "--local-foo", "44", "a"}));
+          {"foo",
+           "--global-foo",
+           "43",
+           "--",
+           "b",
+           "--",
+           "--local-foo",
+           "44",
+           "a"}));
 }
 
 TEST(ProgramOptionsTest, Success) {