Do not add home bin path to PATH if it's already there

This is to allow users to control the order via PATH if they so desire.

Tested by preparing two different `cargo-foo` scripts in `$HOME/.cargo/bin` and `$HOME/bin`:

```
> env PATH="/usr/bin/:$HOME/bin:$HOME/.cargo/bin" ./target/debug/cargo foo
Inside ~/bin/
> env PATH="$HOME/.cargo/bin:/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
> env PATH="/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
```

and trailing slash:

```
> env PATH="$HOME/.cargo/bin/:/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
> env PATH="/usr/bin/:$HOME/bin:$HOME/.cargo/bin/" ./target/debug/cargo foo
Inside ~/bin/
```

Fix rust-lang#11020

tests/testsuite/cargo_command.rs

@@ -2,12 +2,13 @@
 
 use std::env;
 use std::fs;
-use std::io::{Read, Write};
-use std::os::unix::fs::PermissionsExt;
+use std::io::Read;
 use std::path::{Path, PathBuf};
 use std::process::Stdio;
 use std::str;
 
+use cargo_test_support::basic_manifest;
+use cargo_test_support::paths::CargoPathExt;
 use cargo_test_support::registry::Package;
 use cargo_test_support::tools::echo_subcommand;
 use cargo_test_support::{
@@ -346,35 +347,56 @@ fn cargo_subcommand_env() {
 ///
 /// Return paths to both places
 fn set_up_cargo_foo() -> (PathBuf, PathBuf) {
-    let cargo_home_dir = paths::home().join(".cargo");
-    let bin_dir = cargo_home_dir.join("bin");
-    fs::create_dir_all(&bin_dir).unwrap();
-    fs::File::create(&bin_dir.join("cargo-foo"))
-        .unwrap()
-        .write_all(
-            br#"!/usr/bin/env bash
-                echo INSIDE
-            "#,
+    let p = project()
+        .at("cargo-foo")
+        .file("Cargo.toml", &basic_manifest("cargo-foo", "1.0.0"))
+        .file(
+            "src/bin/cargo-foo.rs",
+            r#"fn main() { println!("INSIDE"); }"#,
         )
-        .unwrap();
-
-    fs::File::create(&cargo_home_dir.join("cargo-foo"))
-        .unwrap()
-        .write_all(
-            br#"!/usr/bin/env bash
-                echo OUTSIDE
-            "#,
+        .file(
+            "src/bin/cargo-foo2.rs",
+            r#"fn main() { println!("OUTSIDE"); }"#,
         )
-        .unwrap();
-
-    fs::set_permissions(
-        cargo_home_dir.join("cargo-foo"),
-        fs::Permissions::from_mode(0o555),
-    )
-    .unwrap();
-    fs::set_permissions(bin_dir.join("cargo-foo"), fs::Permissions::from_mode(0o555)).unwrap();
-
-    (cargo_home_dir, bin_dir)
+        .build();
+    p.cargo("build").run();
+    let cargo_bin_dir = paths::home().join(".cargo/bin");
+    cargo_bin_dir.mkdir_p();
+    let root_bin_dir = paths::root().join("bin");
+    root_bin_dir.mkdir_p();
+    let exe_name = format!("cargo-foo{}", env::consts::EXE_SUFFIX);
+    fs::rename(p.bin("cargo-foo"), cargo_bin_dir.join(&exe_name)).unwrap();
+    fs::rename(p.bin("cargo-foo2"), root_bin_dir.join(&exe_name)).unwrap();
+
+    // let cargo_home_dir = paths::home().join(".cargo");
+    // let bin_dir = cargo_home_dir.join("bin");
+    // fs::create_dir_all(&bin_dir).unwrap();
+    // fs::File::create(&bin_dir.join("cargo-foo"))
+    //     .unwrap()
+    //     .write_all(
+    //         br#"!/usr/bin/env bash
+    //             echo INSIDE
+    //         "#,
+    //     )
+    //     .unwrap();
+
+    // fs::File::create(&cargo_home_dir.join("cargo-foo"))
+    //     .unwrap()
+    //     .write_all(
+    //         br#"!/usr/bin/env bash
+    //             echo OUTSIDE
+    //         "#,
+    //     )
+    //     .unwrap();
+
+    // fs::set_permissions(
+    //     cargo_home_dir.join("cargo-foo"),
+    //     fs::Permissions::from_mode(0o555),
+    // )
+    // .unwrap();
+    // fs::set_permissions(bin_dir.join("cargo-foo"), fs::Permissions::from_mode(0o555)).unwrap();
+
+    (root_bin_dir, cargo_bin_dir)
 }
 
 // If `$CARGO_HOME/bin` is not in a path, prefer it over anything in `$PATH`.
@@ -383,7 +405,7 @@ fn set_up_cargo_foo() -> (PathBuf, PathBuf) {
 #[cfg(unix)]
 #[cargo_test]
 fn cargo_cmd_bin_prefer_over_path_if_not_included() {
-    let (_cargo_home_dir, _bin_dir) = set_up_cargo_foo();
+    let (_outside_dir, _inside_dir) = set_up_cargo_foo();
 
     cargo_process("foo").with_stdout_contains("INSIDE").run();
 }
@@ -393,12 +415,12 @@ fn cargo_cmd_bin_prefer_over_path_if_not_included() {
 #[cfg(unix)]
 #[cargo_test]
 fn cargo_cmd_bin_respect_path_when_included() {
-    let (cargo_home_dir, bin_dir) = set_up_cargo_foo();
+    let (outside_dir, inside_dir) = set_up_cargo_foo();
 
     cargo_process("foo")
         .env(
             "PATH",
-            format!("{}:{}", bin_dir.display(), cargo_home_dir.display()),
+            format!("{}:{}", inside_dir.display(), outside_dir.display()),
         )
         .with_stdout_contains("INSIDE")
         .run();
@@ -407,15 +429,15 @@ fn cargo_cmd_bin_respect_path_when_included() {
         // Note: trailing slash
         .env(
             "PATH",
-            format!("{}:{}/", bin_dir.display(), cargo_home_dir.display()),
+            format!("{}:{}/", inside_dir.display(), outside_dir.display()),
         )
         .with_stdout_contains("INSIDE")
         .run();
 
     cargo_process("foo")
         .env(
             "PATH",
-            format!("{}:{}", cargo_home_dir.display(), bin_dir.display()),
+            format!("{}:{}", outside_dir.display(), inside_dir.display()),
         )
         .with_stdout_contains("OUTSIDE")
         .run();
@@ -424,7 +446,7 @@ fn cargo_cmd_bin_respect_path_when_included() {
         // Note: trailing slash
         .env(
             "PATH",
-            format!("{}/:{}", cargo_home_dir.display(), bin_dir.display()),
+            format!("{}/:{}", outside_dir.display(), inside_dir.display()),
         )
         .with_stdout_contains("OUTSIDE")
         .run();