git: Remove use of strings.Builder

This was added in Go 1.10 and is not supported on Go 1.9. Switched to
bytes.Buffer to ensure compatibility.

Signed-off-by: Chris Marchesi <chrism@vancluevertech.com>

Author: vancluever

worktree_commit.go

@@ -1,6 +1,7 @@
 package git
 
 import (
+	"bytes"
 	"path"
 	"strings"
 
@@ -117,7 +118,7 @@ func (w *Worktree) buildCommitSignature(commit *object.Commit, signKey *openpgp.
 	if err != nil {
 		return "", err
 	}
-	var b strings.Builder
+	var b bytes.Buffer
 	if err := openpgp.ArmoredDetachSign(&b, signKey, r, nil); err != nil {
 		return "", err
 	}

worktree_commit_test.go

@@ -1,6 +1,7 @@
 package git
 
 import (
+	"bytes"
 	"strings"
 	"time"
 
@@ -163,7 +164,7 @@ func (s *WorktreeSuite) TestCommitSign(c *C) {
 	// assertStorageStatus(c, r, 1, 1, 1, expectedHash)
 
 	// Verify the commit.
-	pks := new(strings.Builder)
+	pks := new(bytes.Buffer)
 	pkw, err := armor.Encode(pks, openpgp.PublicKeyType, nil)
 	c.Assert(err, IsNil)