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, 6 insertions, 7 deletions

diff --git a/plumbing/format/packfile/diff_delta.go b/plumbing/format/packfile/diff_delta.go
index d35e78a..43f87a0 100644
--- a/plumbing/format/packfile/diff_delta.go
+++ b/plumbing/format/packfile/diff_delta.go
@@ -40,8 +40,8 @@ func getDelta(index *deltaIndex, base, target plumbing.EncodedObject) (plumbing.
 	defer tr.Close()
 
 	bb := bufPool.Get().(*bytes.Buffer)
-	bb.Reset()
 	defer bufPool.Put(bb)
+	bb.Reset()
 
 	_, err = bb.ReadFrom(br)
 	if err != nil {
@@ -49,8 +49,8 @@ func getDelta(index *deltaIndex, base, target plumbing.EncodedObject) (plumbing.
 	}
 
 	tb := bufPool.Get().(*bytes.Buffer)
-	tb.Reset()
 	defer bufPool.Put(tb)
+	tb.Reset()
 
 	_, err = tb.ReadFrom(tr)
 	if err != nil {
@@ -77,6 +77,7 @@ func DiffDelta(src, tgt []byte) []byte {
 
 func diffDelta(index *deltaIndex, src []byte, tgt []byte) []byte {
 	buf := bufPool.Get().(*bytes.Buffer)
+	defer bufPool.Put(buf)
 	buf.Reset()
 	buf.Write(deltaEncodeSize(len(src)))
 	buf.Write(deltaEncodeSize(len(tgt)))
@@ -86,6 +87,7 @@ func diffDelta(index *deltaIndex, src []byte, tgt []byte) []byte {
 	}
 
 	ibuf := bufPool.Get().(*bytes.Buffer)
+	defer bufPool.Put(ibuf)
 	ibuf.Reset()
 	for i := 0; i < len(tgt); i++ {
 		offset, l := index.findMatch(src, tgt, i)
@@ -127,12 +129,9 @@ func diffDelta(index *deltaIndex, src []byte, tgt []byte) []byte {
 	}
 
 	encodeInsertOperation(ibuf, buf)
-	bytes := buf.Bytes()
-
-	bufPool.Put(buf)
-	bufPool.Put(ibuf)
 
-	return bytes
+	// buf.Bytes() is only valid until the next modifying operation on the buffer. Copy it.
+	return append([]byte{}, buf.Bytes()...)
 }
 
 func encodeInsertOperation(ibuf, buf *bytes.Buffer) {