SR-5582 Changed NSString.appendingPathComponent implementation on Linux to match Darwin's.

Fixed following issues where path.appendingPathComponent(component) would produce incorrect result:

path was empty and component did not start with /
or
path ended with / and component started with /

Author: ogres

Foundation/NSPathUtilities.swift

@@ -82,13 +82,13 @@ internal extension String {
     }
 
     internal func _stringByAppendingPathComponent(_ str: String, doneAppending : Bool = true) -> String {
-        if str.length == 0 {
+        if str.isEmpty {
             return self
         }
-        if self == "" {
-            return "/" + str
+        if isEmpty {
+            return str
         }
-        if self == "/" {
+        if hasSuffix("/") {
             return self + str
         }
         return self + "/" + str
@@ -237,16 +237,7 @@ public extension NSString {
     }
 
     internal func _stringByAppendingPathComponent(_ str: String, doneAppending : Bool = true) -> String {
-        if str.length == 0 {
-            return _swiftObject
-        }
-        if self == "" {
-            return "/" + str
-        }
-        if self == "/" {
-            return _swiftObject + str
-        }
-        return _swiftObject + "/" + str
+        return _swiftObject._stringByAppendingPathComponent(str, doneAppending: doneAppending)
     }
 
     public func appendingPathComponent(_ str: String) -> String {

TestFoundation/TestNSString.swift

@@ -77,6 +77,7 @@ class TestNSString : XCTestCase {
             ("test_initializeWithFormat", test_initializeWithFormat),
             ("test_initializeWithFormat2", test_initializeWithFormat2),
             ("test_initializeWithFormat3", test_initializeWithFormat3),
+            ("test_appendingPathComponent", test_appendingPathComponent),
             ("test_deletingLastPathComponent", test_deletingLastPathComponent),
             ("test_getCString_simple", test_getCString_simple),
             ("test_getCString_nonASCII_withASCIIAccessor", test_getCString_nonASCII_withASCIIAccessor),
@@ -752,6 +753,32 @@ class TestNSString : XCTestCase {
             XCTAssertEqual(string, "NSDictionary value is 1000 (42&0)")
         }
     }
+
+    func test_appendingPathComponent() {
+        do {
+            let path: NSString = "/tmp"
+            let result = path.appendingPathComponent("scratch.tiff")
+            XCTAssertEqual(result, "/tmp/scratch.tiff")
+        }
+
+        do {
+            let path: NSString = "/tmp/"
+            let result = path.appendingPathComponent("scratch.tiff")
+            XCTAssertEqual(result, "/tmp/scratch.tiff")
+        }
+
+        do {
+            let path: NSString = "/"
+            let result = path.appendingPathComponent("scratch.tiff")
+            XCTAssertEqual(result, "/scratch.tiff")
+        }
+
+        do {
+            let path: NSString = ""
+            let result = path.appendingPathComponent("scratch.tiff")
+            XCTAssertEqual(result, "scratch.tiff")
+        }                        
+    }
 
     func test_deletingLastPathComponent() {
         do {