Fix binary chunk decoding and inflating

Git uses a unique Base85 encoding with different characters than the
ascii85 encoding implemented by Go, so add a custom decoding function.
Once decoded, use zlib instead of the raw DEFLATE algorithm to
decompress the data.

These issues were caught by some basic parsing tests which are added
here as well.

gitdiff/parser_binary_test.go

@@ -1,6 +1,7 @@
 package gitdiff
 
 import (
+	"encoding/binary"
 	"io"
 	"reflect"
 	"testing"
@@ -108,3 +109,63 @@ func TestParseBinaryFragmentHeader(t *testing.T) {
 		})
 	}
 }
+
+func TestParseBinaryChunk(t *testing.T) {
+	tests := map[string]struct {
+		Input    string
+		Fragment BinaryFragment
+		Output   []byte
+		Err      bool
+	}{
+		"newFile": {
+			Input: "gcmZQzU|?i`U?w2V48*KJ%mKu_Kr9NxN<eH500b)lkN^Mx\n\n",
+			Fragment: BinaryFragment{
+				Size: 40,
+			},
+			Output: fib(10),
+		},
+		"newFileMultiline": {
+			Input: "zcmZQzU|?i`U?w2V48*KJ%mKu_Kr9NxN<eH5#F0Qe0f=7$l~*z_FeL$%-)3N7vt?l5\n" +
+				"zl3-vE2xVZ9%4J~CI>f->s?WfX|B-=Vs{#X~svra7Ekg#T|4s}nH;WnAZ)|1Y*`&cB\n" +
+				"s(sh?X(Uz6L^!Ou&aF*u`J!eibJifSrv0z>$Q%Hd(^HIJ<Y?5`S0gT5UE&u=k\n\n",
+			Fragment: BinaryFragment{
+				Size: 160,
+			},
+			Output: fib(40),
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			p := newTestParser(test.Input, true)
+
+			frag := test.Fragment
+			err := p.ParseBinaryChunk(&frag)
+			if test.Err {
+				if err == nil || err == io.EOF {
+					t.Fatalf("expected error parsing binary chunk, but got %v", err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error parsing binary chunk: %v", err)
+			}
+			if !reflect.DeepEqual(test.Output, frag.Data) {
+				t.Errorf("incorrect binary chunk\nexpected: %+v\n  actual: %+v", test.Output, frag.Data)
+			}
+		})
+	}
+}
+
+func fib(n int) []byte {
+	seq := []uint32{1, 1}
+	for i := 2; i < n; i++ {
+		seq = append(seq, seq[i-1]+seq[i-2])
+	}
+
+	buf := make([]byte, 4*n)
+	for i, v := range seq[:n] {
+		binary.BigEndian.PutUint32(buf[i*4:], v)
+	}
+	return buf
+}