Add ParsePatchHeader and related types (#6)

This function parses patch headers (the preamble returned by the
existing Parse function) to extract information about the commit that
generated the patch. This is useful when patches are an interchange
format and this library is applying commits generated elsewhere.

Because of the variety of header formats, parsing is fairly lenient and
best-effort, although certain invalid input does cause errors.

This also extracts some test utilities from the apply tests for reuse.

Author: bluekeyes

gitdiff/apply_test.go

@@ -6,7 +6,6 @@ import (
 	"io"
 	"io/ioutil"
 	"path/filepath"
-	"strings"
 	"testing"
 )
 
@@ -221,7 +220,7 @@ func (at applyTest) run(t *testing.T, apply func(io.Writer, *Applier, *File) err
 	var dst bytes.Buffer
 	err = apply(&dst, applier, files[0])
 	if at.Err != nil {
-		at.assertError(t, err)
+		assertError(t, at.Err, err, "applying fragment")
 		return
 	}
 	if err != nil {
@@ -238,25 +237,6 @@ func (at applyTest) run(t *testing.T, apply func(io.Writer, *Applier, *File) err
 	}
 }
 
-func (at applyTest) assertError(t *testing.T, err error) {
-	if err == nil {
-		t.Fatalf("expected error applying fragment, but got nil")
-	}
-
-	switch terr := at.Err.(type) {
-	case string:
-		if !strings.Contains(err.Error(), terr) {
-			t.Fatalf("incorrect apply error: %q does not contain %q", err.Error(), terr)
-		}
-	case error:
-		if !errors.Is(err, terr) {
-			t.Fatalf("incorrect apply error: expected: %T (%v), actual: %T (%v)", terr, terr, err, err)
-		}
-	default:
-		t.Fatalf("unsupported expected error type: %T", terr)
-	}
-}
-
 type applyFiles struct {
 	Src   string
 	Patch string

gitdiff/assert_test.go

@@ -0,0 +1,30 @@
+package gitdiff
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func assertError(t *testing.T, expected interface{}, actual error, action string) {
+	if actual == nil {
+		t.Fatalf("expected error %s, but got nil", action)
+	}
+
+	switch exp := expected.(type) {
+	case bool:
+		if !exp {
+			t.Fatalf("unexpected error %s: %v", action, actual)
+		}
+	case string:
+		if !strings.Contains(actual.Error(), exp) {
+			t.Fatalf("incorrect error %s: %q does not contain %q", action, actual.Error(), exp)
+		}
+	case error:
+		if !errors.Is(actual, exp) {
+			t.Fatalf("incorrect error %s: expected %T (%v), actual: %T (%v)", action, exp, exp, actual, actual)
+		}
+	default:
+		t.Fatalf("unsupported expected error type: %T", exp)
+	}
+}