Fix initial commit page & binary munching problem (go-gitea#13249)

Backport go-gitea#13249

* Fix initial commit page

Unfortunately as a result of properly fixing ParsePatch the hack that
used git show <initial_commit_id> to get the diff for this failed.

This PR fixes this using the "super-secret" empty tree ref to make the
diff against.

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Also fix go-gitea#13248

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Update services/gitdiff/gitdiff.go

Co-authored-by: 6543 <6543@obermui.de>

Author: zeripath

services/gitdiff/gitdiff.go

@@ -530,6 +530,8 @@ parsingLoop:
 				break parsingLoop
 			}
 			switch {
+			case strings.HasPrefix(line, cmdDiffHead):
+				break curFileLoop
 			case strings.HasPrefix(line, "old mode ") ||
 				strings.HasPrefix(line, "new mode "):
 				if strings.HasSuffix(line, " 160000\n") {
@@ -850,7 +852,14 @@ func GetDiffRangeWithWhitespaceBehavior(repoPath, beforeCommitID, afterCommitID
 	defer cancel()
 	var cmd *exec.Cmd
 	if (len(beforeCommitID) == 0 || beforeCommitID == git.EmptySHA) && commit.ParentCount() == 0 {
-		cmd = exec.CommandContext(ctx, git.GitExecutable, "show", afterCommitID)
+		diffArgs := []string{"diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M"}
+		if len(whitespaceBehavior) != 0 {
+			diffArgs = append(diffArgs, whitespaceBehavior)
+		}
+		// append empty tree ref
+		diffArgs = append(diffArgs, "4b825dc642cb6eb9a060e54bf8d69288fbee4904")
+		diffArgs = append(diffArgs, afterCommitID)
+		cmd = exec.CommandContext(ctx, git.GitExecutable, diffArgs...)
 	} else {
 		actualBeforeCommitID := beforeCommitID
 		if len(actualBeforeCommitID) == 0 {