Move apply methods to an Applier type

This removes the distinction between "strict" and "fuzzy" application
by allowing future methods on Applier that control settings. It also
avoids state tracking in the text fragment apply signature by moving it
into the Applier type.

While in practice, an Applier will be used once and discarded, the
capability is provided to reset it for multiple uses.

gitdiff/gitdiff.go

@@ -1,6 +1,7 @@
 package gitdiff
 
 import (
+	"errors"
 	"fmt"
 	"os"
 )
@@ -64,6 +65,10 @@ func (f *TextFragment) Header() string {
 // Validate checks that the fragment is self-consistent and appliable. Validate
 // returns an error if and only if the fragment is invalid.
 func (f *TextFragment) Validate() error {
+	if f == nil {
+		return errors.New("nil fragment")
+	}
+
 	var (
 		oldLines, newLines                     int64
 		leadingContext, trailingContext        int64
@@ -117,7 +122,7 @@ func (f *TextFragment) Validate() error {
 
 	// if a file is being created, it can only contain additions
 	if f.OldPosition == 0 && f.OldLines != 0 {
-		return fmt.Errorf("file creation fragment contains context or deletion lines")
+		return errors.New("file creation fragment contains context or deletion lines")
 	}
 
 	return nil