move plumbing from top level package to plumbing (#183)

* plumbing: rename Object -> EncodedObject.

* plumbing/storer: rename ObjectStorer -> EncodedObjectStorer.

* move difftree to plumbing/difftree.

* move diff -> utils/diff

* make Object/Tag/Blob/Tree/Commit/File depend on storer.

* Object and its implementations now depend only on
  storer.EncodedObjectStorer, not git.Repository.
* Tests are decoupled accordingly.

* move Object/Commit/File/Tag/Tree to plumbing/object.

* move Object/Commit/File/Tag/Tree to plumbing/object.
* move checkClose to utils/ioutil.

* move RevListObjects to plumbing/revlist.Objects.

* move DiffTree to plumbing/difftree package.

* rename files with plural nouns to singular

* plumbing/object: add GetBlob/GetCommit/GetTag/GetTree.

1 files changed, 20 insertions, 0 deletions

diff --git a/utils/ioutil/common.go b/utils/ioutil/common.go
index f5b78df..a847abd 100644
--- a/utils/ioutil/common.go
+++ b/utils/ioutil/common.go
@@ -62,3 +62,23 @@ func (writeNopCloser) Close() error { return nil }
 func WriteNopCloser(w io.Writer) io.WriteCloser {
 	return writeNopCloser{w}
 }
+
+// CheckClose is used with defer to close the given io.Closer and check its
+// returned error value. If Close returns an error and the given *error
+// is not nil, *error is set to the error returned by Close.
+//
+// CheckClose is typically used with named return values like so:
+//
+//   func do(obj *Object) (err error) {
+//     w, err := obj.Writer()
+//     if err != nil {
+//       return nil
+//     }
+//     defer CheckClose(w, &err)
+//     // work with w
+//   }
+func CheckClose(c io.Closer, err *error) {
+	if cerr := c.Close(); cerr != nil && *err == nil {
+		*err = cerr
+	}
+}