plumbing: format/packfile, Fix data race and resource leak.

Two problems are fixed:
 - Buffers are not returned to the pool in the case of errors.
 - Per https://golang.org/pkg/bytes/#Buffer.Bytes, the slice returned
   from bytes.Buffer.Bytes() is only valid until the next modification
   of the buffer, so it must be copied before the buffer is returned to the pool.

Running `go test -race` detected the following:

```
==================
WARNING: DATA RACE
Write at 0x00c000224020 by goroutine 15:
  bytes.(*Buffer).WriteByte()
      /usr/local/Cellar/go/1.11.5/libexec/src/bytes/buffer.go:271 +0xc8
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.diffDelta()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/diff_delta.go:95 +0x505
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.getDelta()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/diff_delta.go:60 +0x4ae
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).tryToDeltify()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:309 +0x398
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).walk()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:262 +0x33b
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).ObjectsToPack.func1()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:70 +0x6a

Previous read at 0x00c000224020 by goroutine 8:
  runtime.slicecopy()
      /usr/local/Cellar/go/1.11.5/libexec/src/runtime/slice.go:221 +0x0
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.getDelta()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/memory.go:53 +0x5e2
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).tryToDeltify()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:309 +0x398
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).walk()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:262 +0x33b
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).ObjectsToPack.func1()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:70 +0x6a

Goroutine 15 (running) created at:
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).ObjectsToPack()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:69 +0x761
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*Encoder).Encode()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/encoder.go:52 +0xb1
  vendor/gopkg.in/src-d/go-git%2ev4.pushHashes.func1()
      vendor/gopkg.in/src-d/go-git.v4/remote.go:1026 +0x102

Goroutine 8 (finished) created at:
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*deltaSelector).ObjectsToPack()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/delta_selector.go:69 +0x761
  vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile.(*Encoder).Encode()
      vendor/gopkg.in/src-d/go-git.v4/plumbing/format/packfile/encoder.go:52 +0xb1
  vendor/gopkg.in/src-d/go-git%2ev4.pushHashes.func1()
      vendor/gopkg.in/src-d/go-git.v4/remote.go:1026 +0x102
==================
```

Signed-off-by: Ed Bardsley <ewb@uber.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/plumbing/format/packfile/packfile.go b/plumbing/format/packfile/packfile.go
index f528073..21a15de 100644
--- a/plumbing/format/packfile/packfile.go
+++ b/plumbing/format/packfile/packfile.go
@@ -133,8 +133,8 @@ func (p *Packfile) getObjectSize(h *ObjectHeader) (int64, error) {
 		return h.Length, nil
 	case plumbing.REFDeltaObject, plumbing.OFSDeltaObject:
 		buf := bufPool.Get().(*bytes.Buffer)
-		buf.Reset()
 		defer bufPool.Put(buf)
+		buf.Reset()
 
 		if _, _, err := p.s.NextObject(buf); err != nil {
 			return 0, err
@@ -222,11 +222,11 @@ func (p *Packfile) getNextObject(h *ObjectHeader, hash plumbing.Hash) (plumbing.
 		// optimization only if the expanded version of the object still meets
 		// the small object threshold condition.
 		buf := bufPool.Get().(*bytes.Buffer)
+		defer bufPool.Put(buf)
 		buf.Reset()
 		if _, _, err := p.s.NextObject(buf); err != nil {
 			return nil, err
 		}
-		defer bufPool.Put(buf)
 
 		size = p.getDeltaObjectSize(buf)
 		if size <= smallObjectThreshold {
@@ -321,12 +321,12 @@ func (p *Packfile) fillRegularObjectContent(obj plumbing.EncodedObject) error {
 
 func (p *Packfile) fillREFDeltaObjectContent(obj plumbing.EncodedObject, ref plumbing.Hash) error {
 	buf := bufPool.Get().(*bytes.Buffer)
+	defer bufPool.Put(buf)
 	buf.Reset()
 	_, _, err := p.s.NextObject(buf)
 	if err != nil {
 		return err
 	}
-	defer bufPool.Put(buf)
 
 	return p.fillREFDeltaObjectContentWithBuffer(obj, ref, buf)
 }
@@ -351,12 +351,12 @@ func (p *Packfile) fillREFDeltaObjectContentWithBuffer(obj plumbing.EncodedObjec
 
 func (p *Packfile) fillOFSDeltaObjectContent(obj plumbing.EncodedObject, offset int64) error {
 	buf := bufPool.Get().(*bytes.Buffer)
+	defer bufPool.Put(buf)
 	buf.Reset()
 	_, _, err := p.s.NextObject(buf)
 	if err != nil {
 		return err
 	}
-	defer bufPool.Put(buf)
 
 	return p.fillOFSDeltaObjectContentWithBuffer(obj, offset, buf)
 }