Add basic tests for patch formatting

For now, assert that we produce exactly the same output as the original
parsed input. I think these tests would ideally assert that the
re-parsed value is the same, but it's kind of annoying to compare File
objects without adding an external dependency.

gitdiff/gitdiff_string_test.go

@@ -0,0 +1,33 @@
+package gitdiff
+
+import (
+	"os"
+	"testing"
+)
+
+func TestFile_String(t *testing.T) {
+	sources := []string{
+		"testdata/string/delete.patch",
+		"testdata/string/mode.patch",
+		"testdata/string/modify.patch",
+		"testdata/string/new.patch",
+		"testdata/string/new_empty.patch",
+		"testdata/string/new_mode.patch",
+		"testdata/string/rename.patch",
+		"testdata/string/rename_modify.patch",
+	}
+
+	for _, src := range sources {
+		b, err := os.ReadFile(src)
+		if err != nil {
+			t.Fatalf("failed to read %s: %v", src, err)
+		}
+
+		original := assertParseSingleFile(t, src, b)
+		str := original.String()
+
+		if string(b) != str {
+			t.Errorf("%s: incorrect patch\nexpected: %q\n  actual: %q\n", src, string(b), str)
+		}
+	}
+}

gitdiff/testdata/string/delete.patch

@@ -0,0 +1,16 @@
+diff --git a/file.txt b/file.txt
+deleted file mode 100644
+index c9e9e05..0000000
+--- a/file.txt
++++ /dev/null
+@@ -1,10 +0,0 @@
+-one
+-two
+-three
+-four
+-five
+-six
+-seven
+-eight
+-nine
+-ten

gitdiff/testdata/string/mode.patch

@@ -0,0 +1,3 @@
+diff --git a/file.txt b/file.txt
+old mode 100644
+new mode 100755

gitdiff/testdata/string/modify.patch

@@ -0,0 +1,16 @@
+diff --git a/file.txt b/file.txt
+index c9e9e05..7d5fdc6 100644
+--- a/file.txt
++++ b/file.txt
+@@ -3,8 +3,10 @@ two
+ three
+ four
+ five
+-six
++six six six six six six
+ seven
+ eight
+ nine
+ ten
++eleven
++twelve

gitdiff/testdata/string/new.patch

@@ -0,0 +1,16 @@
+diff --git a/file.txt b/file.txt
+new file mode 100644
+index 0000000..c9e9e05
+--- /dev/null
++++ b/file.txt
+@@ -0,0 +1,10 @@
++one
++two
++three
++four
++five
++six
++seven
++eight
++nine
++ten

gitdiff/testdata/string/new_empty.patch

@@ -0,0 +1,3 @@
+diff --git a/file.txt b/file.txt
+new file mode 100644
+index 0000000..e69de29

gitdiff/testdata/string/new_mode.patch

@@ -0,0 +1,16 @@
+diff --git a/file.sh b/file.sh
+new file mode 100755
+index 0000000..c9e9e05
+--- /dev/null
++++ b/file.sh
+@@ -0,0 +1,10 @@
++one
++two
++three
++four
++five
++six
++seven
++eight
++nine
++ten

gitdiff/testdata/string/rename.patch

@@ -0,0 +1,4 @@
+diff --git a/file.txt b/numbers.txt
+similarity index 100%
+rename from file.txt
+rename to numbers.txt

gitdiff/testdata/string/rename_modify.patch

@@ -0,0 +1,18 @@
+diff --git a/file.txt b/numbers.txt
+similarity index 77%
+rename from file.txt
+rename to numbers.txt
+index c9e9e05..a6b31d6 100644
+--- a/file.txt
++++ b/numbers.txt
+@@ -3,8 +3,9 @@ two
+ three
+ four
+ five
+-six
++  six
+ seven
+ eight
+ nine
+ ten
++eleven