Support adjustable read-ahead in parser

At the moment, we need only to read three lines, but now the value is
easy to adjust as needed. I've only seen the Git implementation read two
lines ahead so far, but it has the whole input in memory and may read
more in other places.

gitdiff/parser.go

@@ -44,7 +44,7 @@ type parser struct {
 
 	eof    bool
 	lineno int64
-	lines  [2]string
+	lines  [3]string
 }
 
 const (
@@ -63,7 +63,7 @@ func (p *parser) ParseNextFileHeader() (file *File, err error) {
 	// based on find_header() in git/apply.c
 
 	for err = p.Next(); err == nil; err = p.Next() {
-		line := p.Line()
+		line := p.Line(0)
 
 		// check for disconnected fragment headers (corrupt patch)
 		if isMaybeFragmentHeader(line) {
@@ -85,16 +85,12 @@ func (p *parser) ParseNextFileHeader() (file *File, err error) {
 		}
 
 		// check for a "traditional" patch
-		if strings.HasPrefix(line, oldFilePrefix) && strings.HasPrefix(p.PeekLine(), newFilePrefix) {
-			if err = p.Next(); err != nil {
-				return nil, err
-			}
-
+		if strings.HasPrefix(line, oldFilePrefix) && strings.HasPrefix(p.Line(1), newFilePrefix) {
 			oldFileLine := line
-			newFileLine := p.Line()
+			newFileLine := p.Line(1)
 
 			// only a file header if followed by a (probable) unified fragment header
-			if !isMaybeFragmentHeader(p.PeekLine()) {
+			if !isMaybeFragmentHeader(p.Line(2)) {
 				continue
 			}
 
@@ -127,9 +123,11 @@ func (p *parser) Next() error {
 	}
 
 	if p.lineno == 0 {
-		// on the first call, need extra shift to initialize both slots
-		if err := p.shiftLines(); err != nil && err != io.EOF {
-			return err
+		// on first call to next, need to shift in all lines
+		for i := 0; i < len(p.lines)-1; i++ {
+			if err := p.shiftLines(); err != nil && err != io.EOF {
+				return err
+			}
 		}
 	}
 
@@ -145,21 +143,20 @@ func (p *parser) Next() error {
 }
 
 func (p *parser) shiftLines() (err error) {
-	p.lines[0] = p.lines[1]
-	p.lines[1], err = p.r.ReadString('\n')
+	for i := 0; i < len(p.lines)-1; i++ {
+		p.lines[i] = p.lines[i+1]
+	}
+	p.lines[len(p.lines)-1], err = p.r.ReadString('\n')
 	return
 }
 
-// Line returns the current line or an empty string if Next has returned io.EOF.
-func (p *parser) Line() string {
-	return p.lines[0]
-}
-
-// PeekLine returns the line following the current line or an empty string if
-// the current line is the final line. If PeekLine returns an empty string,
-// Next will return io.EOF on the next call.
-func (p *parser) PeekLine() string {
-	return p.lines[1]
+// Line returns a line from the parser without advancing it. A delta of 0
+// returns the current line, while higher deltas return read-ahead lines. It
+// returns an empty string if the delta is higher than the available lines,
+// either because of the buffer size or because the parser reached the end of
+// the input. Valid lines always contain at least a newline character.
+func (p *parser) Line(delta uint) string {
+	return p.lines[delta]
 }
 
 // Errorf generates an error and appends the current line information.