Files/FileList: adding the same file twice should not increment FileList::$numFiles

This commit fixes the `FileList::__construct()` and
`FileList::addFile()` methods to ensure that, when there is an attempt
to add the same file twice, the `FileList::$numFiles` variable is not
incremented.

The code was already handling duplicates correctly in the sense that
duplicated files were not added to `FileList::$files`. However,
`FileList::$numFiles` was being incorrectly incremented.

There is some duplicated logic in `FileList::__construct()` and
`FileList::addFile()`. I considered refactoring the duplicated code to a
private method before adding the check that is added in this commit. I
decided not to do it as there are some subtle differences between the
logic in the two methods.

`FileList::__construct()` always sets the value of an entry in the
`FileList::$files` to `null` and the key to whatever is returned by
`SplFileInfo::getPathname()`. While `FileList::addFile()` sets the value
of an entry in the `FileList::$files` to `null` or an object passed to
the method and the key to the path passed to the method.

I decided to leave this refactor to remove the duplication to the future
and focus this commit on fixing the issue with handling duplicated
files.

Author: rodrigoprimo

tests/Core/Files/FileList/ConstructTest.php

@@ -97,6 +97,8 @@ public function testConstruct($files, $expectedFiles)
      */
     public static function dataConstruct()
     {
+        $fixturesDir = __DIR__.DIRECTORY_SEPARATOR.'Fixtures'.DIRECTORY_SEPARATOR;
+
         return [
             'No files'                                     => [
                 'files'         => [],
@@ -113,10 +115,10 @@ public static function dataConstruct()
                 ],
             ],
             'A directory'                                  => [
-                'files'         => [__DIR__.'/Fixtures'],
+                'files'         => [$fixturesDir],
                 'expectedFiles' => [
-                    __DIR__.'/Fixtures/file1.php',
-                    __DIR__.'/Fixtures/file2.php',
+                    $fixturesDir.'file1.php',
+                    $fixturesDir.'file2.php',
                 ],
             ],
             'Same file twice'                              => [
@@ -130,12 +132,12 @@ public static function dataConstruct()
             ],
             'File and then directory containing that file' => [
                 'files'         => [
-                    __DIR__.'/Fixtures/file1.php',
-                    __DIR__.'/Fixtures',
+                    $fixturesDir.'file1.php',
+                    $fixturesDir,
                 ],
                 'expectedFiles' => [
-                    __DIR__.'/Fixtures/file1.php',
-                    __DIR__.'/Fixtures/file2.php',
+                    $fixturesDir.'file1.php',
+                    $fixturesDir.'file2.php',
                 ],
             ],
         ];