On open repository open common cat file batch and batch-check (#15667)

Use common git cat-file --batch and git cat-file --batch-check to
significantly reduce calls to git.
    
Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/tree_nogogit.go

@@ -7,6 +7,8 @@
 package git
 
 import (
+	"io"
+	"math"
 	"strings"
 )
 
@@ -32,6 +34,52 @@ func (t *Tree) ListEntries() (Entries, error) {
 		return t.entries, nil
 	}
 
+	if t.repo != nil {
+		wr, rd, cancel := t.repo.CatFileBatch()
+		defer cancel()
+
+		_, _ = wr.Write([]byte(t.ID.String() + "\n"))
+		_, typ, sz, err := ReadBatchLine(rd)
+		if err != nil {
+			return nil, err
+		}
+		if typ == "commit" {
+			treeID, err := ReadTreeID(rd, sz)
+			if err != nil && err != io.EOF {
+				return nil, err
+			}
+			_, _ = wr.Write([]byte(treeID + "\n"))
+			_, typ, sz, err = ReadBatchLine(rd)
+			if err != nil {
+				return nil, err
+			}
+		}
+		if typ == "tree" {
+			t.entries, err = catBatchParseTreeEntries(t, rd, sz)
+			if err != nil {
+				return nil, err
+			}
+			t.entriesParsed = true
+			return t.entries, nil
+		}
+
+		// Not a tree just use ls-tree instead
+		for sz > math.MaxInt32 {
+			discarded, err := rd.Discard(math.MaxInt32)
+			sz -= int64(discarded)
+			if err != nil {
+				return nil, err
+			}
+		}
+		for sz > 0 {
+			discarded, err := rd.Discard(int(sz))
+			sz -= int64(discarded)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
 	stdout, err := NewCommand("ls-tree", "-l", t.ID.String()).RunInDirBytes(t.repo.Path)
 	if err != nil {
 		if strings.Contains(err.Error(), "fatal: Not a valid object name") || strings.Contains(err.Error(), "fatal: not a tree object") {