plumbing: object, return ErrFileNotFound in FindEntry. Fixes src-d#883

FindEntry will return ErrDirNotFound if the directory doesn't exist. But
it doesn't return the corresponding error if the file itself is missing.
This adds the logic to return ErrFileNotFound, so users can
programmatically check for this condition.

Signed-off-by: James Ravn <james@r-vn.org>

plumbing/object/tree.go

@@ -146,7 +146,11 @@ func (t *Tree) FindEntry(path string) (*TreeEntry, error) {
 		t.t[pathCurrent] = tree
 	}
 
-	return tree.entry(pathParts[0])
+	entry, err := tree.entry(pathParts[0])
+	if err != nil {
+		return nil, ErrFileNotFound
+	}
+	return entry, nil
 }
 
 func (t *Tree) dir(baseName string) (*Tree, error) {

plumbing/object/tree_test.go

@@ -114,6 +114,12 @@ func (s *TreeSuite) TestFindEntry(c *C) {
 	c.Assert(e.Name, Equals, "foo.go")
 }
 
+func (s *TreeSuite) TestFindEntryFileNotFound(c *C) {
+	e, err := s.Tree.FindEntry("not-found")
+	c.Assert(e, IsNil)
+	c.Assert(err, Equals, ErrFileNotFound)
+}
+
 // Overrides returned plumbing.EncodedObject for given hash.
 // Otherwise, delegates to actual storer to get real object
 type fakeStorer struct {