commit: improve error messages

Right now, the error message when trying to parse an invalid tree or
parent header is hard to pin down because it's simply a generic hex
string parsing error.  Make the error easier to discover in the future
by adding some descriptive text to indicate to the user what the problem
is.

Author: bk2204

commit.go

@@ -123,13 +123,13 @@ func (c *Commit) Decode(hash hash.Hash, from io.Reader, size int64) (n int, err
 			case "tree":
 				id, err := hex.DecodeString(fields[1])
 				if err != nil {
-					return n, err
+					return n, fmt.Errorf("error parsing tree: %s", err)
 				}
 				c.TreeID = id
 			case "parent":
 				id, err := hex.DecodeString(fields[1])
 				if err != nil {
-					return n, err
+					return n, fmt.Errorf("error parsing parent: %s", err)
 				}
 				c.ParentIDs = append(c.ParentIDs, id)
 			case "author":