fix: check for error before reading parent (#84)

Co-authored-by: Joe Chen <jc@unknwon.io>

Author: aymanbagabas

repo_diff.go

@@ -45,7 +45,10 @@ func (r *Repository) Diff(rev string, maxFiles, maxFileLines, maxLineChars int,
 				AddOptions(opt.CommandOptions).
 				AddArgs("--full-index", rev)
 		} else {
-			c, _ := commit.Parent(0)
+			c, err := commit.Parent(0)
+			if err != nil {
+				return nil, err
+			}
 			cmd = cmd.AddArgs("diff").
 				AddOptions(opt.CommandOptions).
 				AddArgs("--full-index", "-M", c.ID.String(), rev)
@@ -111,7 +114,10 @@ func (r *Repository) RawDiff(rev string, diffType RawDiffFormat, w io.Writer, op
 				AddOptions(opt.CommandOptions).
 				AddArgs("--full-index", rev)
 		} else {
-			c, _ := commit.Parent(0)
+			c, err := commit.Parent(0)
+			if err != nil {
+				return err
+			}
 			cmd = cmd.AddArgs("diff").
 				AddOptions(opt.CommandOptions).
 				AddArgs("--full-index", "-M", c.ID.String(), rev)
@@ -122,7 +128,10 @@ func (r *Repository) RawDiff(rev string, diffType RawDiffFormat, w io.Writer, op
 				AddOptions(opt.CommandOptions).
 				AddArgs("--full-index", "--no-signature", "--stdout", "--root", rev)
 		} else {
-			c, _ := commit.Parent(0)
+			c, err := commit.Parent(0)
+			if err != nil {
+				return err
+			}
 			cmd = cmd.AddArgs("format-patch").
 				AddOptions(opt.CommandOptions).
 				AddArgs("--full-index", "--no-signature", "--stdout", rev+"..."+c.ID.String())