Handle symlink change event

Hugo 0.16 announced support for symbolic links for the root folders, /content, /static etc., but this got broken pretty fast.

The main problem this commit tries to solve is the matching of file change events to "what changed".

An example:

ContentDir: /mysites/site/content where /mysites/site/content is a symlink to /mycontent

/mycontent:

/mypost1.md
/post/mypost2.md

* A change to mypost1.md (on OS X) will trigger a file change event with name "/mycontent/mypost1.md"
* A change to mypost2.md gives event with name "/mysites/site/content/mypost2.md"

The first change will not trigger a correct update of Hugo before this commit. This commit fixes this by doing a two-step check:

1. Check if "/mysites/site/content/mypost2.md" is within /mysites/site/content
2. Check if  "/mysites/site/content/mypost2.md" is within the real path that /mysites/site/content points to

Fixes #2265
Closes #2273

helpers/path.go

@@ -481,17 +481,17 @@ func SymbolicWalk(fs afero.Fs, root string, walker filepath.WalkFunc) error {
 	}
 
 	// Handle the root first
-	fileInfo, err := lstatIfOs(fs, root)
+	fileInfo, realPath, err := getRealFileInfo(fs, root)
 
 	if err != nil {
 		return walker(root, nil, err)
 	}
 
 	if !fileInfo.IsDir() {
-		return nil
+		return fmt.Errorf("Cannot walk regular file %s", root)
 	}
 
-	if err := walker(root, fileInfo, err); err != nil && err != filepath.SkipDir {
+	if err := walker(realPath, fileInfo, err); err != nil && err != filepath.SkipDir {
 		return err
 	}
 
@@ -511,6 +511,40 @@ func SymbolicWalk(fs afero.Fs, root string, walker filepath.WalkFunc) error {
 
 }
 
+func getRealFileInfo(fs afero.Fs, path string) (os.FileInfo, string, error) {
+	fileInfo, err := lstatIfOs(fs, path)
+	realPath := path
+
+	if err != nil {
+		return nil, "", err
+	}
+
+	if fileInfo.Mode()&os.ModeSymlink == os.ModeSymlink {
+		link, err := filepath.EvalSymlinks(path)
+		if err != nil {
+			return nil, "", fmt.Errorf("Cannot read symbolic link '%s', error was: %s", path, err)
+		}
+		fileInfo, err = lstatIfOs(fs, link)
+		if err != nil {
+			return nil, "", fmt.Errorf("Cannot stat '%s', error was: %s", link, err)
+		}
+		realPath = link
+	}
+	return fileInfo, realPath, nil
+}
+
+// GetRealPath returns the real file path for the given path, whether it is a
+// symlink or not.
+func GetRealPath(fs afero.Fs, path string) (string, error) {
+	_, realPath, err := getRealFileInfo(fs, path)
+
+	if err != nil {
+		return "", err
+	}
+
+	return realPath, nil
+}
+
 // Code copied from Afero's path.go
 // if the filesystem is OsFs use Lstat, else use fs.Stat
 func lstatIfOs(fs afero.Fs, path string) (info os.FileInfo, err error) {