Document and enforce parser invariants

After considering fragment parsing, it made sense to change the
invariant estabilished in the previous commit. Specifically, parser
functions now assume they are call on the first line of their object and
return with the parser on the first line after their object. This means
code can call parse function immediately after each other without
advancing the parser in between.

It's possible this will change back later on... there seem to be
annoying edge cases with either choice, but I think making the functions
consistent is important.

gitdiff/gitdiff.go

@@ -1,6 +1,7 @@
 package gitdiff
 
 import (
+	"fmt"
 	"os"
 )
 
@@ -35,3 +36,8 @@ type Fragment struct {
 	NewPosition int64
 	NewLines    int64
 }
+
+// Header returns the cannonical header of this fragment.
+func (f *Fragment) Header() string {
+	return fmt.Sprintf("@@ -%d,%d +%d,%d @@ %s", f.OldPosition, f.OldLines, f.NewPosition, f.NewLines, f.Comment)
+}