plumbing/object: add Commit.FirstParent

First parents are somewhat special in git.
There's even a --first-parent flag to 'git log'.

Add a helper method to look them up.
This avoids boilerplate and spares the client from
having to arrange for a handle to the Storer,
which is stored in the unexported field Commit.s.

Author: josharian

plumbing/object/commit.go

@@ -3,6 +3,7 @@ package object
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"strings"
@@ -91,6 +92,16 @@ func (c *Commit) NumParents() int {
 	return len(c.ParentHashes)
 }
 
+var ErrNoParents = errors.New("commit has no parents")
+
+// FirstParent returns the first parent of c.
+func (c *Commit) FirstParent() (*Commit, error) {
+	if len(c.ParentHashes) == 0 {
+		return nil, ErrNoParents
+	}
+	return GetCommit(c.s, c.ParentHashes[0])
+}
+
 // File returns the file with the specified "path" in the commit and a
 // nil error if the file exists. If the file does not exist, it returns
 // a nil file and the ErrFileNotFound error.