Issue warning about files with slashes for 'skicka download'

Drive allows file names to have slashes in them, and skicka doesn't handle
these files well. Previously, downloads would silently ignore these files,
which posed the risk of losing data; now at least we issue a warning about
each such file during downloads.

Issue #89.

download.go

@@ -182,6 +182,18 @@ func syncHierarchyDown(driveBasePath string, localBasePath string, trustTimes bo
 		uniqueDriveFiles = files
 	}
 
+	// Warn that we're going to ignore any files that have slashes in their
+	// names.
+	var files []*gdrive.File
+	for _, f := range uniqueDriveFiles {
+		if f.PathHasSlash() {
+			message("%s: skipping file with slash in filename", f.Path)
+		} else {
+			files = append(files, f)
+		}
+	}
+	uniqueDriveFiles = files
+
 	// Create a map that stores the local filename to use for each file in
 	// Google Drive. This map is indexed by the path of the Google Drive
 	// file.

gdrive/gdrive.go

@@ -69,6 +69,8 @@ const metadataVersion = 2
 type File struct {
 	// Path name on Drive. Does not start with a slash.
 	Path string
+	// Indicates whether the original file name in Drive had a slash in it.
+	pathHasSlash bool
 	// Size of the file in bytes.
 	FileSize int64
 	// Unique id of the file (that persists over file modifications,
@@ -137,6 +139,10 @@ func (f *File) driveFile() *drive.File {
 	}
 }
 
+func (f *File) PathHasSlash() bool {
+	return f.pathHasSlash
+}
+
 // IsFolder returns a boolean indicating whether the given File is a
 // folder.
 func (f *File) IsFolder() bool {
@@ -559,11 +565,15 @@ func (gd *GDrive) UpdateMetadataCache(filename string) error {
 			// declaration for an issue with this approach, though.
 			file := new(File)
 			*file = *f
+			file.pathHasSlash = strings.ContainsRune(f.Path, '/')
 			file.Path = p
 
 			gd.pathToFile[p] = append(gd.pathToFile[p], file)
 
-			dir := filepath.Dir(p)
+			// Explicitly trim off the file's name to get the path, rather
+			// than using filepath.Dir(), so that if there is a slash in
+			// the filename, we still get the expected result.
+			dir := filepath.Clean(strings.TrimSuffix(p, f.Path))
 			gd.dirToFiles[dir] = append(gd.dirToFiles[dir], file)
 
 			if f.IsFolder() {