Improve checkIfPRContentChanged (#22611)

The code for checking if a commit has caused a change in a PR is
extremely inefficient and affects the head repository instead of using a
temporary repository.

This PR therefore makes several significant improvements:

* A temporary repo like that used in merging.
* The diff code is then significant improved to use a three-way diff
instead of comparing diffs (possibly binary) line-by-line - in memory...

Ref #22578

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

modules/util/io.go

@@ -4,6 +4,7 @@
 package util
 
 import (
+	"errors"
 	"io"
 )
 
@@ -17,3 +18,24 @@ func ReadAtMost(r io.Reader, buf []byte) (n int, err error) {
 	}
 	return n, err
 }
+
+// ErrNotEmpty is an error reported when there is a non-empty reader
+var ErrNotEmpty = errors.New("not-empty")
+
+// IsEmptyReader reads a reader and ensures it is empty
+func IsEmptyReader(r io.Reader) (err error) {
+	var buf [1]byte
+
+	for {
+		n, err := r.Read(buf[:])
+		if err != nil {
+			if err == io.EOF {
+				return nil
+			}
+			return err
+		}
+		if n > 0 {
+			return ErrNotEmpty
+		}
+	}
+}