Add MLST command in the form of a Get method (#269)

* Add MLST command in the form of a Get method

The `LIST` and `MLSD` commands are inefficient when the objective
is to retrieve one single `Entry` for a known path, because the only way
to get such an entry is to list the parent directory using a data
connection. The `MLST` fixes this by allowing one single `Entry` to be
returned using the control connection.

The name `Get` was chosen because it is often used in conjunction with
`List` as a mean to get one single entry.

Signed-off-by: Thomas Hallgren <thomas@datawire.io>

* Changes in response to code review:

- Rename `Get` to `GetEntry` (because it returns an `*Entry`)
- Add test-case for multiline response on the control connection
- Fix issues with parsing the multiline response.

Signed-off-by: Thomas Hallgren <thomas@datawire.io>

* Add sample output from MLST to GetEntry comment.

Signed-off-by: Thomas Hallgren <thomas@datawire.io>

* Changes in response to code review:

- Remove unused `time.Time` argument
- Add struct labels to make `govet` happy

Signed-off-by: Thomas Hallgren <thomas@datawire.io>

* Remove time arg when calling parseNextRFC3659ListLine

Signed-off-by: Thomas Hallgren <thomas@datawire.io>

Signed-off-by: Thomas Hallgren <thomas@datawire.io>

parse.go

@@ -27,7 +27,11 @@ var dirTimeFormats = []string{
 }
 
 // parseRFC3659ListLine parses the style of directory line defined in RFC 3659.
-func parseRFC3659ListLine(line string, now time.Time, loc *time.Location) (*Entry, error) {
+func parseRFC3659ListLine(line string, _ time.Time, loc *time.Location) (*Entry, error) {
+	return parseNextRFC3659ListLine(line, loc, &Entry{})
+}
+
+func parseNextRFC3659ListLine(line string, loc *time.Location, e *Entry) (*Entry, error) {
 	iSemicolon := strings.Index(line, ";")
 	iWhitespace := strings.Index(line, " ")
 
@@ -35,8 +39,12 @@ func parseRFC3659ListLine(line string, now time.Time, loc *time.Location) (*Entr
 		return nil, errUnsupportedListLine
 	}
 
-	e := &Entry{
-		Name: line[iWhitespace+1:],
+	name := line[iWhitespace+1:]
+	if e.Name == "" {
+		e.Name = name
+	} else if e.Name != name {
+		// All lines must have the same name
+		return nil, errUnsupportedListLine
 	}
 
 	for _, field := range strings.Split(line[:iWhitespace-1], ";") {