Strip spaces from items read via --files-from

In #1590, it was mentioned that while lines read from exclude files via
`--exclude-file` have leading and trailing spaces stripped, this is not
the case for lines read via `--files-from`. This commit fixes that,
spaces are always stripped.

cmd/restic/cmd_backup.go

@@ -304,7 +304,7 @@ func readLinesFromFile(filename string) ([]string, error) {
 
 	scanner := bufio.NewScanner(r)
 	for scanner.Scan() {
-		line := scanner.Text()
+		line := strings.TrimSpace(scanner.Text())
 		// ignore empty lines
 		if line == "" {
 			continue