Add incomplete git header parsing functions

I believe the structure is correct, but there are a lot of details to
fill in - translating the C string parsing logic into Go is not always
straightforward.

gitdiff/gitdiff.go

@@ -1,8 +1,25 @@
 package gitdiff
 
+import (
+	"os"
+)
+
 // File describes changes to a single file. It can be either a text file or a
 // binary file.
 type File struct {
+	OldName string
+	NewName string
+
+	IsNew    bool
+	IsDelete bool
+	IsCopy   bool
+	IsRename bool
+
+	OldMode os.FileMode
+	NewMode os.FileMode
+
+	Score int
+
 	Fragments []*Fragment
 }
 

gitdiff/parser.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"os"
 	"regexp"
 	"strconv"
 	"strings"
@@ -45,7 +46,7 @@ type parser struct {
 const (
 	fragmentHeaderPrefix = "@@ -"
 
-	fileHeaderPrefix = "diff --git"
+	fileHeaderPrefix = "diff --git "
 	oldFilePrefix    = "--- "
 	newFilePrefix    = "+++ "
 )
@@ -127,7 +128,26 @@ func (p *parser) ParseFileChanges(f *File) error {
 }
 
 func (p *parser) ParseGitFileHeader(f *File, header string) error {
-	panic("unimplemented")
+	// TODO(bkeyes): parse header line for filename
+	// necessary to get the filename for mode changes or add/rm empty files
+
+	for {
+		line, err := p.PeekLine()
+		if err != nil {
+			return err
+		}
+
+		more, err := parseGitHeaderLine(f, line)
+		if err != nil {
+			return p.Errorf("header: %v", err)
+		}
+		if !more {
+			break
+		}
+		p.Line()
+	}
+
+	return nil
 }
 
 func (p *parser) ParseTraditionalFileHeader(f *File, oldFile, newFile string) error {
@@ -203,3 +223,108 @@ func parseFragmentHeader(f *Fragment, header string) error {
 
 	return nil
 }
+
+func parseGitHeaderLine(f *File, line string) (more bool, err error) {
+	match := func(s string) bool {
+		if strings.HasPrefix(line, s) {
+			// TODO(bkeyes): strip final line separator too
+			line = line[len(s):]
+			return true
+		}
+		return false
+	}
+
+	switch {
+	case match(fragmentHeaderPrefix):
+		// start of a fragment indicates the end of the header
+		return false, nil
+
+	case match(oldFilePrefix):
+
+	case match(newFilePrefix):
+
+	case match("old mode "):
+		if f.OldMode, err = parseModeLine(line); err != nil {
+			return false, err
+		}
+
+	case match("new mode "):
+		if f.NewMode, err = parseModeLine(line); err != nil {
+			return false, err
+		}
+
+	case match("deleted file mode "):
+		// TODO(bkeyes): maybe set old name from default?
+		f.IsDelete = true
+		if f.OldMode, err = parseModeLine(line); err != nil {
+			return false, err
+		}
+
+	case match("new file mode "):
+		f.IsNew = true
+		if f.NewMode, err = parseModeLine(line); err != nil {
+			return false, err
+		}
+
+	case match("copy from "):
+		f.IsCopy = true
+		// TODO(bkeyes): set old name
+
+	case match("copy to "):
+		f.IsCopy = true
+		// TODO(bkeyes): set new name
+
+	case match("rename old "):
+		f.IsRename = true
+		// TODO(bkeyes): set old name
+
+	case match("rename new "):
+		f.IsRename = true
+		// TODO(bkeyes): set new name
+
+	case match("rename from "):
+		f.IsRename = true
+		// TODO(bkeyes): set old name
+
+	case match("rename to "):
+		f.IsRename = true
+		// TODO(bkeyes): set new name
+
+	case match("similarity index "):
+		f.Score = parseScoreLine(line)
+
+	case match("dissimilarity index "):
+		f.Score = parseScoreLine(line)
+
+	case match("index "):
+
+	default:
+		// unknown line also indicates the end of the header
+		return false, nil
+	}
+
+	return true, nil
+}
+
+func parseModeLine(s string) (os.FileMode, error) {
+	s = strings.TrimSuffix(s, "\n")
+
+	mode, err := strconv.ParseInt(s, 8, 32)
+	if err != nil {
+		nerr := err.(*strconv.NumError)
+		return os.FileMode(0), fmt.Errorf("invalid mode line: %v", nerr.Err)
+	}
+
+	return os.FileMode(mode), nil
+}
+
+func parseScoreLine(s string) int {
+	s = strings.TrimSuffix(s, "\n")
+
+	// gitdiff_similarity / gitdiff_dissimilarity ignore invalid scores
+	score, _ := strconv.ParseInt(s, 10, 32)
+	if score <= 100 {
+		return int(score)
+	}
+	return 0
+}