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

src-d / go-git

plumbing: format/packfile, Fix data race and resource leak. #1159