commit.go: parse whitespace-only lines during continuations

When parsing an extra header that is continued over multiple lines, an
earlier check on the length of whitespace-separated fields caused the
loop to terminate early, dropping continuation lines that consist only
of whitespace.

Tweak the logic slightly in order to capture these, and allow us to
successfully round-trip commit parsing.

Author: ttaylorr

commit.go

@@ -105,7 +105,18 @@ func (c *Commit) Decode(from io.Reader, size int64) (n int, err error) {
 			continue
 		}
 
-		if fields := strings.Fields(text); len(fields) > 0 && !finishedHeaders {
+		if fields := strings.Fields(text); !finishedHeaders {
+			if len(fields) == 0 {
+				// Executing in this block means that we got a
+				// whitespace-only line, while parsing a header.
+				//
+				// Append it to the last-parsed header, and
+				// continue.
+				c.ExtraHeaders[len(c.ExtraHeaders)-1].V +=
+					fmt.Sprintf("\n%s", text[1:])
+				continue
+			}
+
 			switch fields[0] {
 			case "tree":
 				id, err := hex.DecodeString(fields[1])